Radar resolution using monopulse beam sharpening

ABSTRACT

A method and apparatus for improving resolution of targets in a monopulse radar beam.

BACKGROUND OF THE INVENTION

This application is a continuation of and claims priority fromco-pending U.S. patent application Ser. No. 08/856,362 "Radar BasedTerrain And Obstacle Alerting Function" attorney docket number 542-95003filed May 14, 1997, pending, the complete disclosure of which is herebyincorporated herein by reference and which claims priority from U.S.provisional application Ser. No. 60/017,876 filed May 14, 1996.

The present invention relates to monopulse radar and more particularlyto the monopulse radar traditionally used for airborne weatherdetection.

Anticipated increases in air traffic volume, as well as economicpressure to reduce airline operating costs are spurring the developmentof an air transportation system that operates at maximum capacity underall visibility conditions. Several hazards to the safety of flight,however, present themselves or are exacerbated during reduced visibilityoperations. These hazards include: impact with the terrain surroundingthe airport; failure to acquire the intended runway for landing; andfailure to detect obstructions that may be on the runway, taxiway orotherwise in the path of the aircraft. For these reasons, the airtraffic control system imposes minimum cloud ceiling and runwayvisibility requirements at airports that mandate under what conditionsthe airport can accept landing and departing traffic. These minima aredesigned to ensure that the flight crew has enough information toacquire the correct runway and avoid collision hazards on departure andapproach.

In addition, low visibility operations at airports also require that airtraffic control separate landing and departing traffic from each otherby a greater distance. The net effect of the increased separations is toreduce the number of aircraft the airport can handle in a given timeperiod.

Relaxation of the given minima at an airport is possible if both theaircraft and the airport have sophisticated precision guidanceequipment. The precision guidance equipment (e.g. instrument landingsystem (ILS) or microwave landing systems) improves the confidence withwhich the aircraft can acquire and maintain the proper flight path tothe correct runway. Airports having this precision guidance equipmentcan enjoy improved capacity during times of low visibility over airportswithout this equipment. However, this equipment is expensive to acquireand maintain and many airports do not have equipment of this type.Furthermore, these systems require specialized equipment both on boardthe aircraft and at the airport. In addition, use of these systems stilldo not provide the airport with the same capacity present during timesof unrestricted visibility as hazards to flight due to the reducedvisibilities still exist.

Certain dedicated systems are currently manufactured to warn of thesepotential hazards. Chief among these systems are those designed toprevent controlled flight into terrain accidents. Controlled flight intoterrain accidents currently account for the greatest number of airfatalities, the risk of which is greatly increased by operations in lowvisibility conditions. Technology for avoiding controlled flight intoterrain includes ground proximity warning systems, and terrain awarenessand display systems.

Ground proximity warning systems use altitude information from radioaltimeters and barometric altimeters, in conjunction with an individualaircraft's speed and climb characteristics, to warn flight crews thatthe terrain below the aircraft is rising dangerously fast. The groundproximity warning systems can also provide an aircraft flight crew withadditional alerts by, for example, warning of aircraft deviation belowglideslope or inappropriate aircraft attitude or configuration. Typicalexamples of ground proximity warning systems are disclosed in U.S. Pat.No. 3,946,358 entitled "Aircraft Ground Proximity Warning Instrument"and U.S. Pat. No. 4,914,436 entitled "Ground Proximity Approach WarningSystem Without Landing Flap Input," both incorporated herein byreference.

Terrain awareness and display systems combine ground proximity warningsystem technology with navigation data, a built-in terrain data base andexisting cockpit display technology such as color weather radar,electronic flight instrument systems (EFIS) and map displays. Terrainawareness and display systems provide "look ahead" terrain warnings byutilizing present aircraft positions and a terrain data base to predictthe aircraft's future position with respect to terrain. A typicalexample of a terrain awareness system is described in co-pendingapplication Ser. No. 08/509,642, filed Jul. 31, 1995, entitled "TerrainAwareness System" by Muller et al, attorney docket number 543-95-004 andassigned to the same assignee as the present application.

Although the ground proximity warning systems and terrain awareness anddisplay systems described in the above-mentioned references have greatlyreduced the controlled flight into terrain risk for aviation worldwide,both ground proximity warning systems and terrain awareness and displaysystems have some limitations. Neither of these systems actually "sees"the terrain or other obstructions ahead of the aircraft. Groundproximity warning systems differentiate the aircraft's altitude signalsto detect abnormally high closure rates with terrain. Thus,discontinuities in the terrain profiles, such as a ciff, may notgenerate an alert in sufficient time to prevent an accident. The moresophisticated "look ahead" function of terrain awareness and displaysystems compares aircraft position data, based on either dead reckoningor a global positioning system, with a stored terrain map to calculatethe aircraft's probable position relative to the terrain and determinewhether a terrain collision threat exists. However, this system cannotdetect collision threats due to obstructions not contained within thedatabase. For example, temporary structures such as construction craneswould not be modeled in the database. In addition, the integrity of thealerting function depends directly upon the integrity of the aircraftposition data. Errors in aircraft position could reduce the warning timegiven the flight crew. In addition, non-fixed terrain features andnon-fixed terrain threats such as, for example, aircraft or vehiculartraffic on the runway, are also not readily determinable by typicalground proximity warning systems. Thus, these systems are inappropriateas a means for relaxing airport visibility minima and increasing airportcapacities.

Radar has the potential to provide the flight crew with real-timeterrain information independent of both a calculated position and acomputer-stored terrain data base. However, the only radar normallycarried aboard non-military aircraft is weather radar. Weather radar hascharacteristics that make it non optimal for detecting terrain threatsspecifically. Existing weather radar antennas exhibit a limitedelevation sweep angle. The added weight and expense of a radar dedicatedto terrain detection in addition to the already required weather radarprohibits use of terrain only radar systems. Yet, additional safety andincreases in airport capacity could be realized through the use of thisadditional radar information.

Utilizing an aircraft radar for detection of these threats, also posesunique difficulties. Effective real-time terrain feature identificationand terrain threat determination require resolution of closely spacedtargets, for example, closely spaced radio towers. The typical monopulseweather radar antenna transmits through the Sum channel and receivesdata through Sum and Delta channels. Nearly all current radarapplications utilize only these two Sum and Delta channels, which aremanipulated to obtain target off-boresight angle information. Radarapplications utilizing the Sum and Delta channels exclusively areincapable of resolving closely spaced targets. The monopulse measurementis accurate only when a single target is present in the radar beam. Ifmultiple targets are present in the beam or if the target is widelydistributed, the monopulse measurement becomes confused: the Sum channelshows broadening but no distinction between targets, the Delta channelshows broadening and a null point between targets but shows noseparation between targets. Traditional monopulse angle measurement isunable to separate two closely spaced targets; known monopulsesharpening techniques may even degrade the image. Thus, it has beendifficult to distinguish multiple targets or widely distributed targetswhen they are concurrently present in the radar beam. Such targets arelikely to be present in the vicinity of an airport.

Furthermore, the weather radar typically operates at a wavelengthoptimized to reflect small droplets of water. This wavelength presentsadditional problems when attempting to resolve large and/or closelyspaced targets.

SUMMARY OF THE INVENTION

The present invention enhances the ability of monopulse radars to detectand resolve targets. The present invention enables the monopulse radarstraditionally used in a variety of applications to resolve closelyspaced targets, such as radio towers, multiple targets, and widelydistributed targets.

According to one aspect of the present invention, the invention may beimplemented using the monopulse weather radar typically used foraircraft weather detection. Use of the weather radar in conjunction withthe present invention enables the weather radar to be employed to detectterrain and obstacle targets and avoids the cost and weight penaltiesassociated with using a separate dedicated terrain radar system.

According to another aspect of the present invention, the monopulse beamsharpening function utilizes a new distributed monopulse channel,Delta₋₋ D, which is more sensitive to signal to interference than thetraditionally used Delta channel, thereby improving the resolution ofmultiple targets in the radar beam. The invention can thus distinguishbetween the multiple targets, or obstructions. In a preferred embodimentof the invention, the multiple targets are distinguished through timemultiplexing the traditional monopulse channel with the Delta₋₋ Dchannel to provide more accurate angle measurement for targets in eachside of the beam.

Other features and advantages of the present invention are described ingreater detail below. Although the present invention is described belowin connection with an aircraft weather radar and specific applicationstherefor, it should be readily understood by those of skill in the artthat the present invention may be used with any monopulse radar andassociated applications to improve target resolution. Such applicationsmay include, but are not limited to, for example, the use of shipboardradar to detect harbor features, rocks, and other vessels.

Similarly, the present invention supplements the Traffic Alert andCollision Avoidance System by providing warnings of potential runway ormid-air collisions, even when the intruding aircraft is not equippedwith an Air Traffic Control Radar Beacon System transponder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a illustrates the use of the autonomous landing guidance in atypical airport scenario according to one embodiment of the presentinvention;

FIG. 1b illustrates the use of the autonomous landing guidance in atypical airport scenario according to one embodiment of the presentinvention;

FIG. 1c illustrates the use of the autonomous landing guidance in atypical airport scenario according to one embodiment of the presentinvention;

FIG. 2 is a top level system block diagram of an autonomous landingguidance function according to one embodiment of the present invention;

FIG. 3 is a system block diagram of an autonomous landing guidancefunction according to one embodiment of the present invention;

FIG. 4 shows the transmit/receive waveform utilized in a preferredembodiment of the autonomous landing guidance mode of the presentinvention;

FIG. 5 illustrates an example of a stored features data base accordingto one embodiment of the present invention;

FIG. 6 illustrates a signal processing algorithm for terrain andobstacle detection function and azimuth position sharpening functionaccording to one embodiment of the present invention;

FIG. 7a illustrates narrow bandwidth radar resolution of dominant radiotower scatterers;

FIG. 7b illustrates radar resolution of dominant radio tower scatterersusing wide bandwidth radar according to one embodiment of the presentinvention;

FIG. 8a illustrates a valid range gate having a signal-to-noise ratiowhich exceeds a preselected threshold setting according to oneembodiment of the present invention;

FIG. 8b illustrates the angle of arrival of a valid range gate accordingto one embodiment of the present invention;

FIG. 8c illustrates both the elevation extent or height above terrainand range extent for a valid range gate according to one embodiment ofthe present invention;

FIG. 9 illustrates the azimuth beam sharpening concept according to oneembodiment of the present invention;

FIG. 10a illustrates antenna scan angle in degrees versus relativeamplitude for both a real-beam image and a monopulse sharpened imageaccording to one embodiment of the present invention;

FIG. 10b illustrates translation errors in between a reference image anda radar derived image according to an embodiment of the presentinvention;

FIG. 10b illustrates autocorrelation of the reference image andcorrelation of the reference image with the radar image;

FIG. 10c illustrates correlation of the reference image with radargenerated image;

FIG. 11 shows an example of autonomous landing guidance mode data asdisplayed on a Heads-up Display according to one embodiment of thepresent invention;

FIG. 12 illustrates the terrain and obstacle detection modesignal-to-noise ratio for terrain and landmarks with intervening rainaccording to one embodiment of the present invention;

FIG. 13 illustrates the signal-to-clutter ratios for various runwayintrusion targets according to one embodiment of the present invention;

FIG. 14 shows the autonomous landing guidance system height measurementperformance curve in a typical airline airframe for terrain with anintervening rain versus range according to one embodiment of the presentinvention;

FIG. 15 shows a signal processing algorithm for a runway intrusiondetection function according to one embodiment of the present invention;

FIG. 16 illustrates the thresholding and acceptance logic function ofthe terrain and obstacle detection function according to one embodimentof the terrain and obstacle detection function of the present invention;

FIG. 17a illustrates the interleaving of weather, windshear, andautonomous landing guidance mode data in a condition where windshear isnot present according to one embodiment of the present invention;

FIG. 17b illustrates the interleaving of weather, windshear, andautonomous landing guidance mode data in a condition where windshear ispresent according to one embodiment of the present invention;

FIG. 18 is a functional block diagram for the autonomous landingguidance radar function according to one embodiment of the presentinvention;

FIG. 19 illustrates one embodiment of radar antenna system according toone embodiment of the present invention;

FIG. 20 illustrates the functional diagram of the radar solid statetransmitter according to one embodiment of the present invention;

FIG. 21 illustrates a functional diagram of the radar exciter accordingto one embodiment of the present invention;

FIG. 22 illustrates a functional diagram of the radar radio frequencyreceiver according to one embodiment of the present invention;

FIG. 23 illustrates a functional diagram of the radar intermediatefrequency receiver according to one embodiment of the present invention;

FIG. 24 illustrates a functional diagram of the radar timing generatoraccording to one embodiment of the present invention;

FIG. 25 illustrates a functional diagram of the radarpreprocessor/digital pulse compressor according to one embodiment of thepresent invention;

FIG. 26 is a functional diagram of the processing function that performsfrequency domain pulse compression according to one embodiment of theinvention;

FIG. 27 illustrates a functional diagram of the radar digital prefilteraccording to one embodiment of the invention;

FIG. 28 illustrates a functional diagram of the radar power accumulationand saturation count function according to one embodiment of theinvention;

FIG. 29 illustrates the traditional Sum and Delta patterns for a typical32-inch flat plate antenna array with a 3.2 degree beam width used inweather radar;

FIG. 30 is an illustrative analysis of a point target response in orderto illustrate the basic relationships of the Sum and Delta channels;

FIG. 31 illustrates the distributed monopulse channel when two targetsappear simultaneously in the beam;

FIG. 32 is a vector representation of the two-point target case;

FIG. 33a illustrates monopulse angle measurement techniques according toone embodiment of the invention;

FIG. 33b illustrates off-boresight angle measurements according to anembodiment of the present invention;

FIG. 34 is an illustrative example comparing two off-boresight angles,the traditional monopulse and the distributed monopulse, for a singlepoint target;

FIG. 35 is an illustrative example comparing two off-boresight angles,the traditional monopulse and the distributed monopulse, for the case oftwo identical in-phase targets that are separated by 1 degree;

FIG. 36 is an illustrative example comparing a real-beam image with atraditional monopulse sharpened image for the case of two equal sizein-phase targets spaced apart by 1 degree;

FIG. 37 illustrates a real-beam image compared with the image resultingfrom combining the traditional monopulse and the distributed monopulseaccording to the monopulse beam sharpening mode invention for two equalsize in-phase targets spaced apart by 1 degree;

FIG. 38 illustrates a real-beam image compared with the image resultingfrom combining the traditional monopulse and the distributed monopulseaccording to the monopulse beam sharpening mode invention for twoidentical targets spaced apart by 1 degree and 90 degrees out-of-phase;

FIG. 39 illustrates antenna scan patterns according to an embodiment ofthe present invention;

FIG. 40a-c illustrate terrain and display formats according to anembodiment of the present invention;

FIG. 41 illustrates the interleaving of terrain and weather dataaccording to an embodiment of the present invention;

FIG. 42 illustrates the maximum vertical climb with 25 g accelerationversus warning distance.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the invention are described herein in thecontext of an Autonomous Landing Guidance System for aircraft useful forexplaining the features and operation of the present invention. Those ofordinary skill in the art will recognize that the invention may beadopted for use in other contexts.

1.0 Autonomous Landing Guidance System

FIG. 1a contains an illustration of a typical airport facility usefulfor explaining use of the autonomous landing guidance invention. When anaircraft 24 is on glide slope 26 to runway 28, the invention employsradar imaging to measure terrain features at or in the vicinity of theairport and creates an image of the radar-detected terrain. For example,terrain features might include: a building 30, a control tower 32, aterminal 34, hangers 36, 38 and hill 39.

The radar-created image is compared with identifying features retrievedfrom a terrain and airport data base; relevant portions of which areaccessed using aircraft position information. Correlating the data baseinformation with the radar image can be used to independently identifythe airport and to calculate the aircraft's position relative to runway28, including the aircraft's orientation relative to runway 28. Thecorrelated radar and aircraft position data is then used to update theaircraft's position relative to the airport and to provide the flightcrew with the updated position information via an aircraft display. In apreferred embodiment, this display is a heads-up display but mayoptionally be the weather radar display or EFIS. Thus, the flight crewknow the aircraft's position relative to the runway touch-down point 40and relative to the various stationary terrain obstacles, for example,building 30, control tower 32, terminal 34, hangers 36, 38 and hill 39.An intrusion detection function, to be described below, scans for anytransient obstacles in the flight path of the aircraft, such as runwayintrusions or other aircraft and alerts the flight crew in the event anintrusion is detected.

FIG. 1a may now be used in conjunction with FIGS. 1b-1c to explain theoperation of the landing system in greater detail. In a typical airportscenario, when the aircraft 24 is on the glide slope 26 the inventionuses radar in the form of a coherent monopulse beam to measure terrain,for example, building 30 height, control tower 32 height, terminal 34and hangers 36, 38 at or in the vicinity of the airport 42. In apreferred enbodiment, the radar is an X-band radar typically used forairborne weather detection. The invention creates an image of all radardetected objects above a selected clearance plane 44 using terrainobstacle detection measurements and azimuth monopulse beam sharpening toenhance the radar image as will be described in greater detail below.

FIG. 2 shows a top level system block diagram of an autonomous landingguidance function 10 according to one embodiment of the presentinvention. A position reference function 12 provides a calculatedaircraft position based on position information developed, for example,by the aircraft's on-board global positioning system receiver and/orinertial measurement unit. Other sources of navigation inputs known tothose skilled in the art, may be used to provide navigation data to theposition reference function. The aircraft's calculated position is usedto access a pre-stored reference profile 46 of airport 42 terrain andobstacle heights in stored data base of airport identifying features 14.Pre-stored reference profile 46 of airport terrain and obstacle heightsis then correlated against the radar-created image. The outputs of thecorrelation function confirm the airport location and provide azimuth,elevation, and range error corrections to update the aircraft position.The invention also identifies the runway area 28, runway orientation,and runway centerline from the pre-stored map. Guidance commands areissued to adjust the aircraft's position relative to the glide slope andrunway touch down point. The intrusion detection feature also functionsto detect transient obstacles in the aircraft flight path using highrange resolution processing and monopulse discrimination.

1.1 Autonomous Landing Guidance Function System Definition

FIG. 3 is a system block diagram of an autonomous landing guidancefunction 100 according to one embodiment of the present invention. Aposition reference function 102 uses input from an on-board globalpositioning system, for example, a GPS receiver 104, and/or an inertialmeasurement unit (IMU) 106 to compute range to touch-down, line-of-sightvelocity, and acceleration. Other sources of navigation inputs known tothose skilled in the art may be used to provide navigation data toposition reference 102. Position reference function 102 is also used toaccess a stored features map 108. Stored feature map 108 references areobtained from surveys or map data showing elevation extended landmarksat or in the vicinity of airports, for example, towers, tall buildings,and hangers. The landmark locations relative to each runway touch-downpoint are stored in feature map 108 for each airport.

A terrain and obstacle detection function 110 includes a monopulse radarsystem to be described in greater detail later. Terrain and obstacledetection function 110 accesses aircraft position information stored inposition reference 102. Monopulse azimuth position function 112 providesenhanced azimuth position information to terrain and obstacle detectionfunction 110. A feature extraction function 114 extracts terrainfeatures and terrain obstacles at and in the vicinity of the airportfrom the monopulse enhanced radar image of the terrain and obstacledetection function 110.

Features are extracted based on their height above an elevationclearance plane which is adjusted to yield the desired number of terrainfeatures. The invention utilizes approximately three to six landmarks tocharacterize any airport and perform the feature correlation function116. Selection of a clearance plane limits the terrain featuresconsidered by feature correlation function 116 to those landmarks whoseheight causes them to extend above the chosen clearance plane andeliminates from consideration any terrain feature not tall enough toextend above the clearance plane. Limiting the number of terrainfeatures considered in feature correlation function 116 through use of aclearance plane reduces a minimum the number of features which willallow successful airport identification.

Feature correlation function 116 correlates the radar extracted featuresagainst pre-stored features for the destination airport. If at least a90% correlation exists between extracted and pre-stored features, anairport identification confirmation function 118 independently confirmsthe airport identification and precisely identifies the runway locationat the airport. Once the runway area is identified, airportidentification confirmation function 118 communicates runway location tothe aircraft's Head-up Display and to a navigation error measurement orcorrection function 120.

Navigation error correction function 120 updates the position referenceinformation and communicates updated guidance commands or positionerrors to the Head-up Display to facilitate adjustment of the aircraft'sposition relative to the glide slope and runway touch down point.

An intrusion detection function 122 searches for any intruding targets,for example, aircraft or other vehicular traffic on the runway.Intrusion detection function 122 processes the same data used by theterrain and obstacle detection function 110 and provides intrusion alertinformation to the Head-up Display and to an intrusion warning device,for example, an audio warning system.

The present autonomous landing guidance system invention uses a 6 kHzpulse repetition frequency (PRF) to provide a 6.7 nautical mileunambiguous range which is considered adequate for terrain featureextraction for purposes of the autonomous landing guidance function. Thenumber of range bins is selected to cover a minimum of 8 kilometers. Theoptimum waveform has a 6 kHz pulse repetition frequency and the numberof range bins is selected to cover a minimum unambiguous range of 8kilometers. Table 1 defines radar parameter values according to apreferred embodiment of the invention.

FIG. 4 shows the transmit/receive waveform 130 utilized in a preferredembodiment of the autonomous landing guidance mode of the invention. Thetransmit/receive waveform dwell includes 128 pulses, collected over 4frequency sub-bands 132, each having 8 different frequencies. The 32frequencies are pulse-to-pulse multiplexed to eliminate the effects ofsecond time around echoes (STAE) and interference from other aircraftradar signals.

1.2 Reference Image

Feature extraction function 114 extracts terrain features from the radarimage to characterize an airport and perform feature correlationfunction 116. An example of airport characterization function accordingto the invention is shown in FIG. 4. Stored features data base 108includes a terrain reference image generated by surveys of elevationextended landmarks including, for example, tall building 30, controltower 32, and hangers 36, 38 in the vicinity of airport 42 and eachlandmark's location relative to each runway touchdown point 40, 140.Stored features data base 108 includes similar terrain reference imagesof elevation extended landmarks which characterize individual airportsthroughout the world. In particular, FIG. 5 illustrates an example of astored features data base including terrain reference images generatedby surveys of elevation extended landmarks including, hangers 36, 38,building 30, and control tower 32, in the vicinity of a specific airportand each landmark's location relative to each runway touchdown point 40,140.

Landmark locations are stored in 3-dimensional coordinates according toany technique known to those of skill in the art. In the example of FIG.5, the location 142 of a first touchdown point 40 corresponds to (x,y,z)coordinates (0,0,0). The location of a second touchdown point 140corresponds to (x,y,z) coordinates (tpx, tpy, tpz). The locations 144,146 of hanger landmarks 36, 38 correspond to (x,y,z) coordinates (lx1,ly1, lz1) and (lx2, ly2, lz2), respectively. The location 148 ofbuilding landmark 30 corresponds to (x,y,z) coordinates (lx3, ly3, lz3).The location 150 of control tower landmark 32 corresponds to (x,y,z)coordinates (lx4, ly4, lz4).

The invention utilizes approximately three to six landmarks tocharacterize any airport and perform the feature correlation functionwith a 99.9% probability of correctly confirming the airportidentification while limiting the probability of false confirmation to 1in 1 million. The probability of the invention being unable to eithercorrectly or falsely confirm the airport identification is 1 in 1,000.

Position reference function 102 uses input from global positioningsystem receiver 104 and/or inertial measurement unit 106 to compute anabsolute global position, range to touchdown, line-of-sight velocity,and acceleration. The absolute global position is used to access thelandmark reference data stored in stored features data base 108.Position reference function 102 then calculates the aircraft's positionrelative to stored landmark locations and uses the results to updateposition, range to touchdown, line-of-sight velocity, and accelerationinformation. The absolute and relative position data are calculated andup-dated at an update rate of 47 Hz.

1.3 Feature Extraction

Feature extraction function 114 is used to identify landmarks that areabove a selected elevation clearance plane 44. The height of elevationclearance plane 44 is selected to yield the desired number of features.As noted above, approximately 6 to 10 features are selected forcorrelation with reference data stored in stored features function 108.The peak height for each valid range gate of each dwell is applied tothreshold and acceptance logic function 182 which thresholds the peakheight to determine whether the range bin is above the selectedclearance plane 44. The invention codes the thresholded data into threecategories: below buffer zone, above buffer zone, and unknown. Thethresholded data is also used by intrusion detection function 122 toissue ground collision avoidance warnings.

1.4 Feature Correlation, Airport Confirmation and Runway Orientation

Feature correlation function 116 correlates the terrain featuresextracted from the radar image against pre-stored features for thedestination airport. The correlation function compares the prestoredreference image with the radar generated image to determine translationerrors. Rotation and scaling errors are assumed to be small.

Translation errors ocur in both range and cross-range. FIG. 10b shows anexample of translation errors between a reference image 117a and a radarderived image 117b. FIG. 10c shows the auto-correlation of the refernceimage, which has no translation error, and the correlation of thereference image with the radar image, which shows translation errors inrange and cross range. Note that the unmatched radar data and referencedata themselves have little or no impact on the correlation outcome.Furthermore, the correlation limit is ±10 positions in range and crossrange.

Airport identification function 118 evaluates output of featurecorrelation function 114 to determine whether a sufficiently strongmatch exists between the radar profile and the stored airport reference.A correlation index is computed based on the degree of correlation,scan-to-scan variation and residual errors. When the correlation indexis above a preselected level, for example, 90%, the invention uses therunway orientation, width, and other characteristics stored in the database to generate a display template in the Head-up Display format asshown in FIG. 11. The Head-up Display format shows the runway 240, therunway touchdown point 242, and the runway centerline 244. The displaytemplate is transmitted for display on the aircraft's Head-up Display.Optionally, the invention also transmits the computed correlation indexto the aircraft's sensor fusion processor. The sensor fusion processormay be used to integrate the synthetic image generated by the Head-upDisplay format display template with the aircraft's other sensor imageson multifunction displays.

1.5 Navigation Error Computation

Following completion of airport identification function 118, positiondata generated by position reference function 102 of FIG. 3 calculatesthe aircraft's position relative to the stored landmark location. Theresults are transmitted to navigation error measurement function 120which computes navigation errors relative to runway touch down point142. The navigation errors computed include: range, azimuth angle,elevation angle, and velocity. In a preferred embodiment of theinvention, these navigation errors and any other computed navigationerrors are transmitted to the aircraft's sensor fusion processor tofacilitate display of position and runway alignment information to thepilot. The pilot can then use the visual display of alignment data tomaintain or correct the aircraft flight path. Optionally, the navigationerrors may be provided as input to an autoland or other flight controlsystem of the type known to those of skill in the art. Examples of thedesign of such systems are described in Examples of the design of suchsystems are described in "Aircraft Control and Simulation" by BrianStevens and published by John Wiley and Sons and "Automatic Control ofAircraft and Missiles" by John H. Blakelock also published by John Wileyand Sons.

1.6 Terrain and Obstacle Detection Function

Terrain and obstacle detection function 110 of the present inventionprovides real-time situational awareness by providing a terrain radarimage which defines the terrain ahead of the aircraft, including mappedand unmapped obstacles. The terrain and obstacle detection function usesthe weather radar to image both natural terrain and man-made terrain andprovide obstacle warnings. The radar-imaged terrain includes naturalterrain, including unmapped new terrain created by earth movements, andman-made terrain, for example, radio towers, hangers, control towers, orother buildings, and vehicular traffic. The radar-imaged terrain can beused for runway and airport identification as described above and/or forterrain and obstacle avoidance.

Because the aircraft is moving relative to the terrain, terrain andobstacle detection function 110 must account for Doppler shift. TheDoppler shift in pulsed radar is manifested in the phase of target echosignals from hit to hit. In order to recover the Doppler shift, thesystem measures and records the phase of each received echo. Samples ofthe target's position and amplitude may be gathered in by directsampling or I/Q sampling. Although radar target echo signals may besampled by direct sampling, the sense of the Doppler shift, theinformation as to whether the target was closing or moving away, islost. Thus, most radar recover the Doppler shift using l/Q sampling togather the target's position and amplitude information, where "I" standsfor in-phase and is the cosine or real component of the signal and "Q"stands for quadrature and is the sine or imaginary component of thesignal. The method of l/Q recovery is well known to those of skill inthe art.

FIG. 6 shows a signal processing algorithm 152 for terrain and obstacledetection function 110 and azimuth position sharpening function 112according to one embodiment of the present invention. Additionalembodiments of the terrain and obstacle detection mode are discussed indetail later. Signal processing algorithm 152 includes multiple inputports for receiving l/Q samples from a l/Q demodulator and aircraftvelocity data and aircraft acceleration data from appropriate navigationsensors and applies the various data to a motion compensator/datade-multiplexor 154. The motion compensation phase of motioncompensator/data de-multiplexor 154 removes the Doppler shift caused byaircraft motion. The data de-multiplexor phase demodulates the radarsignals for local oscillator, LO, phase coding. The phase shift causedby aircraft motion at each pulse repetition interval is given by:##EQU1## where: c=speed of light;

F=radio frequency in Hz;

i=pulse repetition interval number=1, 2, . . . 128; and

ΔR=change in range since the beginning of the dwell.

    ΔR=V.sub.los *ΔT×i+0.5(A.sub.los *ΔT×l).sup.2Eq (2)

ΔT=1/pulse repetition frequency;

V_(los) =line-of-sight velocity; and

A_(los) =line-of-sight acceleration.

The input data is motion compensated and phase demodulated according to:

    CX.sub.i =X.sub.i ×e.sup.i(Φ.sbsp.i.sup.+Φ.sbsp.c)Eq (3)

where:

X_(i) =complex radar data;

Φ_(c) =local oscillator phase modulation code 0, π, . . . ; and

CX_(i) =corrected complex data.

The complex data is then de-multiplexed to form Sum and Delta channelsaccording to:

    S(n,f) and D(n,f)

where:

n=coherent pulse repetition interval number 1, 2, . . . , 16;

f=frequency number 1, 2, . . . , 8;

S=SUM channel data; and

D=Delta channel data.

Motion compensator/data de-multiplexor 154 includes a Sum channel outputand a Delta channel output. The Sum channel data is applied to a first2:1 Sum channel coherent integrator 156 where the complex data for eachfrequency and each range gate are coherently integrated over 16 pulserepetition intervals according to: ##EQU2## The Delta channel data isapplied to a second 2:1 Delta channel coherent integrator 158 where thecomplex data for each frequency and each range gate are coherentlyintegrated over 16 pulse repetition intervals according to: ##EQU3## Thecoherent integration is used for gain over noise and cancellation ofcomplementary code sidelobes. The output of the Sum channel coherentintegrator 156 is applied to a square law detector 160 which uses theSum channel coherent integrator 156 data to compute SS* and DD* for usein the monopulse angle calculation according to:

    SS*=(S.sub.i +jS.sub.Q)×(S.sub.i -jS.sub.Q)=S.sub.i.sup.2 +S.sub.Q.sup.2 ; and                                      Eq (6)

    DD*=(D.sub.i +jD.sub.Q)×(D.sub.i -jD.sub.Q)=D.sub.i.sup.2 +D.sub.Q.sup.2.                                           Eq (7)

The Sum channel coherent integrator 156 includes a second output whichis applied to intrusion detection function 122. Signal processingalgorithm 152 includes a product detector 162. The outputs of both Deltachannel coherent integrator 158 and Sum channel coherent integrator 156are applied to the product detector 162 which combines the signals tocalculate Real DS*! and Imaginary DS! used in angle measurementaccording to:

    Re DS*!=S.sub.i ×D.sub.i +S.sub.Q ×D.sub.Q ; andEq (8)

    Im DS*!=S.sub.i ×D.sub.Q -S.sub.Q ×D.sub.i.    Eq (9)

Square law detector 160 output, SS* and DD*, and the product detector162 output data, Real DS*! and Im DS*!, is applied to a 8:1 postdetection integrator 164. Post detection integrator 164 providesnon-coherent integration of SS* and DD* and Real DS*! and Im DS*! overthe eight frequencies of the four frequency subbands of the modewaveform. ##EQU4## Post detection integrator 164 data is manipulated byan elevation extent function 166 to calculate an elevation extent wherethe elevation extent or height above terrain of vertically extendedtargets, for example, a tower, is estimated from the standard deviationof the monopulse measurement over the multiple frequencies. The weightedextent is computed according to: ##EQU5## where: σ=target extent; and

kext=extent factor nominal value=1.7.

An average centroid angle function 168 calculates an average centroidangle using the output of post detection integration 164. The centroidangle and a reference angle are input to an azimuth monopulse positionfunction 170 to determine an azimuth monopulse position. The resultingazimuth angle is provided to feature extraction function 114. Thetarget's off-boresight angle, μ, is directly proportional to the ratioReal DS*!/SS* and is calculated according to: ##EQU6## where: kslope=anempirically determined antenna slope factor.

1.6.1 Radio Frequency Bandwidth

The bandwidth of the transmitted signal determines the range resolutionperformance and frequency agility capabilities of the radar. Rangeresolution is the ability to separately detect multiple targetsappearing simultaneously in the beam. Range resolution is a function ofthe radar radio frequency signal bandwidth. Generally, wide bandwidthsallow resolution of more closely spaced targets. Range resolutionrequires the targets to be separated by a larger range when the pulsewidth is long. Range resolution of multiple targets requires that theindividual targets be separated by at least the range equivalent of thewidth of the processed echo pulse. Although range resolution may beenhanced using pulse compression, which eliminates the trade-off of widepulses for the high energy needed for target detection against narrowpulses for range resolution, range resolution can be generally definedby signal bandwidth according to the relationship:

    ΔR≈c/(2B),                                   Eq (14)

where:

ΔR=the range resolution in meters;

c=the velocity of propagation in meters per second; and

B=the echo waveform's matched bandwidth in Hertz.

Transmit/receive waveform dwell 130 shown in FIG. 4 includes 128 pulses,collected over 4 frequency sub-bands each having 8 differentfrequencies. The number of frequencies and the spacing between them areselected to satisfy three criteria: having a minimum number of samplesor frequencies such that angle averaging results in true spatialaveraging; maximizing non-coherent gain from frequency agility toproduce smooth averaging; and having a statistically high number ofsamples of frequencies such that the estimate of elevation extent orheight above terrain is accurate.

The autonomous landing guidance invention uses frequency agility toreduce amplitude scintillation of backscattering from distributedtargets. Thus, if N observations of randomly distributed targets aremade at N different frequencies separated by a minimum delta frequency,ΔF, then the average of the N measurements is statistically equivalentto the average of N spatially independent observations of thedistributed target. In other words, frequency agility is used to reduceglint and the fading variance just as spatial averaging would. Wehner,Donald R., High Resolution Radar, Artech House, Inc., Norwood, Mass.,1987, incorporated herein by reference, shows that the greatest gainfrom frequency agility is obtained from the first 6 to 10 independentsamples. Therefore, the present invention uses 8 frequencies in apreferred embodiment. The minimum frequency spacing ΔF is given by:

    ΔF=c/2ΔR,                                      Eq (15)

where:

c=speed of light; and

ΔR=range depth of the narrowest target of interest.

For example, if monopulse measurements are desired on very narrow, forexample, ≦20 feet, but high radio towers, then the minimum frequencyspacing is 20 MHz according to the relationship of range resolution tosignal bandwidth shown above.

A radio tower is made visible to radar imaging by scatterers placed atvarious points amid its structure, each of which acts as a target toreflect the radar signal. A narrow beamwidth is unable to resolve theclosely packed scatterers into individual targets. The result is anaveraging of the echoes from the several individual tower scattererswhich appear simultaneously in the beam. Thus, a narrow beamwidth canmeasure the average height of, for example, a very narrow but high radiotower, but cannot accurately measure the radio tower's elevation extentor height above terrain.

FIG. 7a illustrates that a narrow bandwidth 173 radar cannot resolvedominant radio tower scatterers 174 of a tall, very narrow target 176equipped with radar scatterers. Narrow beamwidth radar 173 measuresaverage height but cannot measure elevation extent or height aboveterrain which limits effective resolution. FIG. 7b illustrates that,conversely, a wide bandwidth radar 178, constructed according to theinvention, effectively resolves radio tower 176 into individual dominantradio tower scatterers 174 and accurately measures the average centroidangle and elevation extent or height above terrain of radio tower 176 toa 3-foot effective resolution. The total frequency agile bandwidthselected for detecting and measuring the radio tower is thenapproximately 160 MHz. This bandwidth effectively samples the towerscatterers to approximately 1 meter range resolution which enables theradar to detect the top of high narrow radio towers.

1.6.2 Top of Terrain Estimate

A top-of-target angle calculation is performed by signal processingalgorithm 152 of FIG. 6. The top-of-target angle is equal to the averagecentroid angle plus the elevation extent or height above terrain. Thetop-of-target angle is added to the horizontal referenced elevationangle, which is computed based on the antenna elevation angle and thetilt angle, to compute the target elevation angle below horizontal. Inaddition, the off-boresight angle is roll compensated according to:

    β.sub.top =φ.sub.ref +(μ+σ)×cos (ρ)Eq (16)

where:

φ_(ref) =horizontal referenced antenna elevation pointing angle,provided at a minimum rate of 100 Hz; and

ρ=roll angle.

The top-of-target angle is then applied to a sliding window averager 180which is also an M-of-N detector. A target angle measurement is validaccording to the M-of-N detector of the invention if the target anglemeasurement is detected in at least 2 of 4 frequency sub-bands whereeach dwell covers approximately 1 degree of antenna scan. Therefore, thetypical aircraft weather radar antenna with a 3-degree beam width willscan past a target in three dwells. The M-of-N sliding window averager180 is used to improve probability of detection and reduce false alarmrate.

The SS*, convolution data, centroid, extent and noise level data areapplied to a thresholding and acceptance logic function 182 whichvalidates each range gate of each dwell. A valid range gate satisfiesthree separate criteria: (1) a signal-to-noise ratio which exceeds apreselected threshold setting 184 as illustrated in FIG. 8a; (2) anangle of arrival within the antenna linear monopulse region 186 of FIG.8b; and (3) Both the elevation extent or height above terrain and rangeextent are consistent with a valid target as illustrated in FIG. 8c.

Several examples showing application of the latter criterion areillustrated in FIG. 8c. In a first example, a range bin 188 includingterrain 190 is accepted because terrain 190 exhibits a very lowelevation extent. In a second example, a range bin 192 including a radiotower 194 is accepted because tower 194 exhibits a high elevation extentor height above terrain and a low range extent. In another example,range bins 196, 198, 200, 202, 204 including heavy rain 206 will berejected by thresholding and acceptance logic function 182 because heavyrain 206 exhibits a combination of high elevation extent and large rangeextent considered inconsistent with a valid target.

A height computation function 208 estimates the highest point in eachrange gate, peak angle, β_(peak), by peak detecting over two dwells thedata computed by the sliding window averager 180 according to:

    β.sub.peak =max β.sub.top (n)!                   Eq (17)

where:

n=dwell number 1, 2 . . .

Height computation function 208 then converts the peak angle calculatedby a peak angle computation function 210 into target height bymultiplying the peak angle by the range. The results of heightcomputation function 208 are output to feature extraction function 114.

1.6.3 Azimuth Monopulse Sharpening

Monopulse sharpening techniques improve target azimuth position accuracyover the real-beam imaging techniques described above. FIG. 9illustrates the azimuth beam sharpening concept of the presentinvention. In FIG. 9 beam scan direction 212 is indicated by the arrow.Azimuth monopulse angle measurements of a distributed target 214 areused to accurately determine the azimuth position of significant radarreflectors. The Sum beam scan 216 of the scanned space is divided intosmall angular bins 218, 220, 222, . . . of 0.3 degrees or less, thentarget monopulse angles 224, 226, 228, . . . are weighted based on theintensity at that specific scan direction and a statistical probabilityof target extent is calculated based on the number of measurements 230appearing in angular bins 218, 220, 25 222, . . .

The azimuth off-boresight angle of a target is given by: ##EQU7## Theazimuth off-boresight angle is then added to the reference antennaazimuth angle and quantized to determine the appropriate azimuth angularbin according to:

    α=φ.sub.REF +μ.sub.az ×cos (ρ)      Eq (19)

    i=nint(α/d)                                          Eq (20)

where:

d=azimuth bin size=0.3 degrees;

i=quantized azimuth angle; and

nint=nearest integer value.

The peak elevation angle for the coherent processing interval (CPI) isweighted by the average power and then positioned in the appropriateazimuth bin:

    β(i)=β(i)+β.sub.peak ×SS.sub.az       Eq (21)

The total power in each azimuth angular bin is accumulated according to:

    P(i)=P(i)+SS.sub.az                                        Eq (22)

The average peak angle is computed according to:

    β(i)=β((i)/P(i))                                 (Eq (23)

FIG. 10a, discussed in greater detail later, illustrates antenna scanangle in degrees versus relative amplitude for both a monopulsesharpened image 232 and a real-beam image 234. FIG. 10 illustrates theimprovement resulting from monopulse beam sharpening, according to theinvention, over real-beam image 234.

Optionally, other techniques for monopulse beam sharpening known tothose of skill in the art may be used. An alternative monopulse beamsharpening technique according to another emodiment of the invention isdescribed later in this specification.

1.6.4 Clutter-to-Noise Ratio

FIG. 12 shows the terrain and obstacle detection mode clutter-to-noiseratio for -35 dB square meter per square meter terrain 274 and 100square meter landmarks 276 with a 12 millimeter per hour interveningrain when using an autonomous landing guidance radar system conformingto the expanded system parameters of Table 2. The radar cross-section ofmost landmarks of interest, buildings, hangars and towers, are expectedto be much larger than 100 square meters. FIG. 13 illustrates thesignal-to-clutter (S/C) ratios for 1 square meter runway intrusiontarget 278 and a 10 square meter runway intrusion target 280. Assuming aSwirling type 1 target with runway backscatter of -50 dB square meterper square meter and grass backscatter of -50 dB square meter per squaremeter, the signal-to-clutter ratio for a 90% single look probability ofdetection (Pd₁) is 15 dB. The probability of detection usinglook-to-look correlation is given by:

    PD.sub.2 =1-(1-Pd.sub.1).sup.2.                            Eq (24)

Look-to-look correlation is used to provide a high probability ofdetection while minimizing the probability of a false alarm.

1.6.5 Measurement Accuracy

When absolute height measurements are required, for example, in a groundcollision avoidance situation, the radar measurement errors areconsidered in addition to all other alignment errors in order to predictthe overall system performance. The ground collision avoidance situationis discussed later. When the terrain and obstacle detection function isused to extract high elevation landmarks, only the relative heightmeasurement is generally utilized for collision avoidance. All commonerrors, for example, static and low frequency dynamic errors, cangenerally be ignored since such errors only bias height measurements upor down. The process of adaptive elevation cuts, discussed above atfeature extraction function 114, allows selection of as many features asrequired to perform the correlation with the stored reference. Theheight measurement accuracy provides correlation even on features thatare not highly extended, for example, features extending as little as 10meters above the surrounding terrain.

FIG. 14 shows the autonomous landing guidance system height measurementperformance curve 282 in a typical airline airframe for -35 dB squaremeter per square meter terrain with a 12 millimeter per hour interveningrain versus range in kilometers when using a system conforming to thesystem parameters of Table 2. In most radar applications nearly allheight error is due to mechanical misalignment errors rather than radarmonopulse angle measurement errors. Therefore, more accurate heightmeasurement performance may be obtained for a specific application byperforming more accurate boresight corrections on the radar after it ismounted.

1.6.6 Intrusion Detection

Intrusion detection function 122 of FIG. 3 provides the flight crew withreal-time situational awareness by providing warnings of obstacles inthe aircraft's path. The runway area is normally clear of targets orobstructions. If intrusion detection function 122 detects an image abovethe threshold, an alert is generated.

FIG. 15 shows a signal processing algorithm 260 for runway intrusiondetection function 122. Intrusion detection function 122 receives asinput the same data used by terrain and obstacle detection function 110.Terrain and obstacle detection function 110 is executed when the radarantenna is in the vicinity of the runway.

(Range resolution may be enhanced using pulse compression, whicheliminates the trade-off of wide pulses for the high energy needed fortarget detection against narrow pulses for range resolution. Pulsecompression is done on waveforms which contain modulation, commonlyfrequency modulation (FM), within the pulses. The modulation is afrequency sweep across the pulse, called "chirp." Thus, improved rangeresolution is obtained by processing the frequency chirp data.

Specifically, the radar Sum channel output is applied to an 8 point(8-PT) fast Fourier transform (FFT) function 262 coupled to a downstreamsquare law detector 264, while the Delta channel output is applied to asecond 8-PT fast Fourier transform function 265 coupled to a downstreamproduct detector 267. The output of both square law detector 264 andproduct detector 267 are processed by a 4:1 post detection integrator288. An azimuth and elevation monopulse angle function 270 computesazimuth and elevation monopulse angles and a two-dimensional constantfalse-alarm rate (CFAR) 272 is generated. Constant false-alarm ratefunction 272 sets and applies the detection threshold, according to:

    CFAR=P.sub.FA *R.sub.DT,                                   Eq (25)

where:

P_(FA) =the probability of false alarm is the probability thatinterference alone will cross the threshold for a look or compound test;and

R_(DT) =the rate at which detection tests occur, 1/s is equal to thebandwidth at the point of the test and may be the same as the range binrate.

Constant false alarm-rate function 152 decides whether or not a targetis present. The azimuth and elevation angles, two-dimensional constantfalse-alarm rate 272, and runway location developed by airportidentification and confirmation function 118 are output or provided tointrusion detector 122. Intrusion detector 122 compares the signalsagainst a preselected threshold and generates an intrusion alert signalif appropriate.

1.6.7 Probability of Detection and False Alarm

According to one preferred embodiment, the invention uses look-to-lookcorrelation to provide a probability of detection of an intruding targetof 99.99% and a probability of false alarm of 1×10⁻⁶ at the output ofterrain and obstacle detection warning function 110. The signal-to-noiseratio satisfying these levels of detection probability and false alarmprobability is obtained by determining the desired level of probabilityof detection and probability of false alarm at the thresholding as shownin FIG. 16. The M of N sliding window effect on probability of detectionand probability of false alarm is given by: ##EQU8## where: P=inputprobability;

Po=output probability;

n=size of the window; and

m=minimum number of hits out of n.

As indicated in FIG. 16, thresholding and acceptance logic function 182has probability of detection equal to 0.97 and a probability of falsealarm equal to 4×10⁻⁴ based on Sum power and noise level inputs. Anoutput probability of 99.99% results from an input probability of 99%. Afalse alarm probability of 1×10⁻⁶ at the output of the M of N detectorresults from an probability of false alarm of 6×10⁻⁴ at the input.

In a situation where terrain clutter behaves as a Swerling case 2target, and a number of independently fluctuation reflectors of aboutequal size, a clutter-to-noise ratio of approximately 8.0 dB satisfies aprobability of detection and probability of false alarm at the output ofthe terrain and obstacle detection warning function 48 of 99.99% and1×10⁻⁶, respectively, when using a system conforming to the systemparameters of Table 2 and look-to-look correlation. FIG. 12 shows that aclutter-to-noise ratio of approximately 8.0 dB may be obtained with a 12millimeter per hour intervening rain at a range of 8 kilometers whenusing a radar system conforming to the system parameters recited inTable 2.

1.7 Mode of Operation

There are various ways of implementing the autonomous landing guidancemode of the present invention. According to one preferred embodiment ofthe invention, the present invention uses an X-band radar traditionallyused for weather detection and avoidance. When the present invention isimplemented using weather radar, compliance must be maintained withcurrent Federal Aviation Administration regulations requiring weatherand windshear detection capability. The windshear detection function istypically enabled below 1,200 feet altitude, in the vicinity of anairport during take off and landing. In addition, the integrity of theradar in weather gathering function should be maintained. The autonomouslanding guidance information, according to the present invention, may beinterleaved with both the weather information and the windshearinformation. Various methods of interleaving autonomous landing guidanceinformation with weather and windshear information are possible. Theinterleaving of autonomous landing guidance information with weatherinformation and windshear information according to one embodiment of theinvention is explained below.

FIG. 17a illustrates the interleaving of weather, windshear, andautonomous landing guidance mode data in a condition where windshear isnot present. As shown in FIG. 17a, during the first scan 284 of theradar system, weather data is collected and during the second scan 286windshear data is collected. In a condition where no windshear isdetected, the invention uses a third scan 288 and fourth scan 290 tocollect autonomous landing guidance mode terrrain data.

FIG. 17b illustrates radar operation in the presence of windshear. InFIG. 17b weather data is collected during the first scan 284 of theradar system and windshear data is collected during the second scan 286.When windshear is detected during second scan 286, the inventioncollects windshear data during a third scan 292 and fourth scan 294followed by a fifth scan 296 during which autonomous landing guidancemode terrain data is collected. In one preferred embodiment, theautonomous landing guidance mode data collection and processing isaccomplished over approximately 2 seconds. During the autonomous landingguidance mode azimuth scan the antenna elevation position is computedbased on the aircraft's attitude. The antenna scans a selected azimuthsector centered on the aircraft velocity vector while collecting azimuthand elevation monopulse data at a selected scan rate, for example, theantenna scans a 90 degree azimuth scan sector at 45 degrees per secondsuch that each dwell is collected over approximately 1 degree scancoverage. If intruding targets are detected, the invention displays theintruding targets as previously described.

1.8 Hardware Implementation of Autonomous Landing Guidance System

Table 2 recites the autonomous landing guidance radar system parametersfor one preferred embodiment of the present invention. According to oneembodiment of the present invention, the radar sensor transmits andreceives at frequencies in the general range of 9.32 GHz to 9.48 GHzusing a solid state transmitter having on the order of 160 watts peakpower. The radar sensor directivity gain is on the order of 35 dB. Theradar sensor minimum radio frequency bandwidth is 160 MHz with 32equally spaced frequency steps. The radar sensor pulse width is from 100nanoseconds to 512 microseconds. The radar sensor duty cycle is in thegeneral range of 0.06% to 15%. The radar sensor noise figure is 5 dB orless. The radar sensor minimum range resolution capability is 15 m. Theradar sensor azimuth angular resolution is on the order of 3 degrees andthe elevation angular resolution is on the order of 4 degrees. The radarsensor is capable of performing analog-to-digital conversion of radardata.

1.8.1 Autonomous Landing Guidance Mode Radar Functional Block Diagram

FIG. 18 is a functional block diagram for the autonomous landingguidance radar function 300 according to one embodiment of the presentinvention. In FIG. 18, autonomous landing guidance radar function 300includes a modified conventional monopulse-type single-antennatransmitter/receiver weather radar of a type well known in the artoperating at a predetermined pulse repetition frequency (PRF).Autonomous landing guidance radar function 300 includes a conventionalpulse repetition frequency timing generator 310, an exciter 320, a solidstate transmitter 340, antenna 350, a circulator/duplexer 370, a generalpurpose processor 380, a monopulse radio frequency receiver 390, anintermediate frequency receiver 400, a preprocessor/digital pulsecompressor 420, a digital signal processor 450, display units 460, andexternal interfaces 470. Autonomous landing guidance radar function 300is powered by external power supplies 480.

Timing generator 310 generates the frequencies and synchronizationsignals used by the radar system which determines when transmitter 340fires. Exciter 320 activates transmitter 340 to initiate the emission ofthe radar pulses which are fed through to the radar antenna 350. At thesame time that timing generator 310 pulses exciter 320 to initiate theemission of radar pulse, timing generator 310 also produces outputpulses for simultaneous application to preprocessor/digital pulsecompressor 420 and to general purpose processor 380 to determine how theother radar system functions relate to the time of transmission.

Exciter 320, which is explained later in greater detail, translates thewaveform to the radar's illumination frequency and amplifies thewaveform to a level usable by the final power amplifier portion oftransmitter 340. Exciter 320 supplies a transmitter drive signal totransmitter 340; a second output to radio frequency receiver 390, acalibration signal to the microwave integrated circuit (MIC) locatedbehind antenna 350; and multiple local oscillator output signals tointermediate frequency receiver 400.

Transmitter 340 produces a short radar frequency pulse throughcirculator/duplexer 370 to antenna 350 at a pulse repetition frequencydetermined by a synchronous pulse generator. The pulsing of transmitter340 causes a radar carrier signal pulse to be transmitted from radarantenna 350. Transmitter 340 employs frequency agility or multiplefrequency transmission, whereby the carrier frequency is changed frompulse to pulse.

Antenna 350 is a standard 30-inch flat plate antenna array with a 3.2degree beam width typically used in weather radar operating in theX-band and receiving pulse echoes for transmission through the Sum andDelta channels of the monopulse receiver. Antenna 350 includes bothazimuth and elevation sector scan capability. The electromagnetic energyradiated from antenna 350 travels through space until striking theterrain or other obstacle. Reflected radar pulses are received byantenna 350 and sent via the isolating device to the receiver. Antenna350 is energized via a signal from solid state transmitter 340 under thecontrol of timing generator 310 via exciter 320 to radiate pulses in asuitable frequency band.

Circulator/duplexer 370 functions as an isolating device whichalternately connects antenna 350 of the monopulse single-antenna systemto transmitter 340 and radio frequency receiver 390 and isolates antenna350 from the non-connected function. Circulator/duplexer 370 alsoprotects radio frequency receiver 390 from transmitted power.Circulator/duplexer 370 includes three ports: transmitter 340, antenna350, and radio frequency receiver 390. Optionally, circulator/duplexer370 may have a fourth port between transmitter 340 and receiver 390. Thefourth port is terminated which increases the path loss.Circulator/duplexer 370 also functions in conjunction with a localoscillator (not shown) to translate the signal and interference to theintermediate frequency which is the difference between the signalfrequency and that of the local oscillator. The resulting intermediatefrequency is fed to the radio frequency receiver 390. The outputs ofantenna 350 Delta azimuth channel and Delta elevation channel are alsofed to radio frequency receiver 390 and through to intermediatefrequency receiver 400. Analog-to-digital converters 402 associated withintermediate frequency receiver 400 transform the analog I and Q signalsto digital words for use by digital signal processor 450.

General purpose processor 380 controls the attitude of antenna 350through manipulation of servo motors via a gimbal drive mechanism and aposition feedback mechanism. Both the gimbal control mechanism and theposition feedback mechanism are responsive to drive and error signalsfrom general purpose processor 380. Antenna 350 horizontal scan positioninformation and vertical scan position information, azimuth andelevation signals respectively, are supplied to general purposeprocessor 380 from the position feedback mechanism. The positionfeedback mechanism includes azimuth and elevation angular rate sensorsand synchronous read-outs such as is well known to those of skill in theart, for example, see U.S. Pat. No. 3,369,231. The position feedbackmechanism may include one or more accelerometers for sensinglongitudinal and transversal antenna acceleration if antenna 350 phasecenter stability requirement cannot be otherwise achieved. According tothe invention, the gimbal drive motors and the position feedbackmechanism also rotate the antenna array 352 between a first attitude anda second attitude perpendicular to said first attitude. The gimbal drivemotors and the position feedback mechanism cause the antenna to scan inazimuth and elevation and may also provide space stabilization ofantenna array 352, as is well understood by those skilled in the art.General purpose processor 380 is coupled to the vertical and horizontalindicators of the display units 460 through external interfaces 470.General purpose processor 380 converts the azimuth and elevationsignals, indicative of horizontal and vertical position, respectively,for use by display units 460.

Monopulse radio frequency receiver 390 is coupled to the multiple feedsof antenna 350 and provides a monopulse Sum and Difference, video signaloutput. Radio frequency receiver 390 Sum channel receives the input fromcirculator/duplexer 370 in the form of additively combined energy fromthe two lobes of the radiation pattern of antenna 350 symmetrical aboutantenna 350 center line and mutually angulated in elevation. Theelevation Difference channel, or Delta channel, of radio frequencyreceiver 390 receives an input in the form of differentially combinedpulse echoes which is received in the two lobes of the radiation patternof antenna 350 symmetrical about antenna 350 center line and mutuallyangulated in azimuth. Radio frequency receiver 390, which is explainedin greater detail later, includes a power limiting section 392, a lownoise amplification section 394 an image rejection filtering section 396and a down-conversion section 398 coupled to receive one output, LO1, ofexciter 320. The output of radio frequency receiver 390 is anintermediate frequency signal which is transmitted to intermediatefrequency receiver 400.

Intermediate frequency receiver 400, which is explained in greaterdetail later, includes an intermediate frequency blanking section 404coupled to a first down-conversion and gain control section 406 coupledin turn to a second down-conversion and gain control section 408 coupledto matched filters 410 which are coupled to a coherent detector 412.Coherent detector 412 is coupled to associated analog-to-digitalconverters 402. The output of intermediate frequency receiver 400 istransmitted to preprocessor/digital pulse compressor 420.

Preprocessor/digital pulse compressor 420 performs digital pulsecompression on the range gate samples to provide enhanced rangeresolution as is well known in the art. See Byron Edde, Radar:Principles, Technology, Applications, Prentice Hall PTR, 1993, Chapter13; pages 23-4, which is incorporated by reference. Preprocessor/digitalpulse compressor 420, which is explained in greater detail later,includes a pulse compression section 422 where a matched filter functioncorrelates the echo wave with a delayed copy of the transmitted signal;a prefilter section 424 which generates the desired matched filterbandwidth; and a data storage section 426 which performs a data storagefunction which provides a place to keep digitized signals temporarilywhile all the signals for a particular process are gathered. Digitalpulse compressor 420 is explained in greater detail in a later section.

A digital signal processor 450, as is well known in the art, processestarget echoes and interfering signals to increase target echo signallevel and suppress interference, thereby increasingsignal-to-interference ratio. Digital signal processor 450 also performsthe detection function, making the decision as to whether or not atarget is present, and recovers information about targets, for example,position, range, and Doppler shift. Digital signal processor 450 is usedin the invention to synthesize matched filters for the various radarapplications. Digital signal processor 450 is explained later in greaterdetail.

Display units 460 having a video output are coupled topreprocessor/digital pulse compressor 420 through digital signalprocessor 450 and to external interfaces 470. Preprocessor/digital pulsecompressor 420 also receives the output of timing generator 310. Digitalsignal processor 450 controls the information on display units 460 as afunction of (range from the aircraft to the object. When an object isdetected, the color corresponding to a range bracket to that object willbe generated on the display by means well known to those of skill in theart. See, for example, U.S. Pat. No. 3,369,231, which is incorporated byreference.

1.8.2 Antenna System

FIG. 19 illustrates one embodiment of radar antenna system 350 accordingto the present invention. Radar antenna system 350 includes an antennaarray 352, drive motors 354 and position feedback mechanism 356.Position feedback mechanism 356 includes synchronous read-outs 358 andboth azimuth and elevation angular rate gyroscopes 360. Antennaacceleration sensors 362 may be employed to monitor both longitudinaland transversal antenna acceleration when antenna phase center stabilitycannot otherwise be satisfactorily achieved. Drive motors 354 receiveazimuth angle and elevation angle rate commands from general purposeprocessor 380 and mechanically drive antenna array 352. Synchronousread-outs 358 feedback antenna array 352 azimuth and elevation positiondata to general purpose processor 380. Azimuth and elevation angularrate gyroscopes 360 feedback antenna 350 azimuth and elevation angularrate data to general purpose processor 380. If present, antennaacceleration sensors 362 feedback both antenna longitudinal accelerationand antenna transversal acceleration to general purpose processor 380.General purpose processor 380 utilizes the position, angular rate andacceleration data to control antenna array 352 position according totraditional control methods. The preferred antenna positioner istwo-axes stabilized with a minimum scan coverage of ±90 degrees inazimuth and +50 to -65 degrees in elevation. The antenna positionerincludes a azimuth sector scan which is selectable from 0 to ±90 degreesand an elevation sector scan which is selectable from 0 to 115 degrees.The antenna positioner scan rate in both azimuth and elevation isvariable on the order of 0 to 60 degrees per second with minimum gimbalacceleration and deceleration on the order of 600 degrees per second persecond. A preferred embodiment of the antenna positioner conforms to theperformance parameters of Table 3. Antenna 350 includes an antenna arraywhich conforms to the performance parameters of Table 4.

1.8.3 Transmitter Functional Diagram

FIG. 20 illustrates the functional diagram of solid state transmitter340 according to one embodiment of the present invention. Transmitter340 receives a transmitter drive control signal from exciter 320. Thetransmitter drive control signal has a 9400 MHz center frequency withina ±80 MHz band at +12 dBm, nominal. Transmitter 340 receives atransmitter gate control signal from timing generator 310. Transmitter340 transmits the radio frequency power to antenna 350 at 52.2 dBm.

Transmitter 340 includes a driver amplifier section 342, a radiofrequency power amplifier section 344, a power combiner section 346, anda modulator section 348. Driver amplifier section 342 receives andamplifies the transmitter drive control signal from exciter 320. Thecircuit of transmitter 340 achieves parallelism by dividing the drivepower among multiple amplifying modules and combining the resultingpower onto a single transmission line as a radio frequency output at theoutput of power combiner section 346 for application tocirculator/duplexer 370. Preferred transmitter 340 conforms to theperformance parameters of Table 5.

Modulator 348 controls the transmitter circuit. Modulator 348 ispreferably of a type known in the art as a low-level modulator.Modulator 348 responds to an 120 volt RMS input signal at 400 Hz,nominal. An output gate of modulator 348 provides isolated terminals toturn outputs on/off by asserting a logic signal. Modulator 348 supplies1152 watts total power at 144 amperes and 8 volts. Modulator 348distributes the power into eight individual 16-ampere outputs and twoindividual 8-ampere outputs. Modulator 348 supplies a gate voltage at -2volts and 1 ampere, total, distributed to ten individual outputs.Modulator 348 includes built-in-test (BIT) capability and reportsbuilt-in-test status by asserting a logic signal.

Modulator 348 supports pulse widths from 100 nanoseconds to 512microseconds and exhibits a maximum voltage drop across a 512microsecond pulse width of 0.5 volts. Modulator 348 includes a switchingcapacitor for each power module in order to support gracefuldegradation. The maximum output random noise of modulator 348 is 4millivolts RMS measured at a 25 MHz bandwidth. maximum output amplituderipple of modulator 348 is 20 millivolts peak-to-peak. Theinput-to-output isolation voltage of modulator 348 is on the order of700 volts direct current, the input-to-case isolation voltage is on theorder of 500 volts direct current, and the output-to-case isolationvoltage is on the order of 300 volts direct current. Modulator 348provides normal output with standard 180 volts alternating current for aminimum of 0.1 seconds and the output voltage returns to normal limitswithin 100 microseconds after a 50% change in load current. Modulator348 has a maximum load transient over/under shoot of 0.5 volts from anominal voltage set point. Each output of modulator 348 is independentlyprotected against a short circuit of any duration and automaticallyrestores to normal when an overload is removed. The maximum temperaturecoefficient of modulator 348 is 0.03% per degree centigrade.

1.8.4 Exciter Functional Diagram

Exciter 320 translates the waveform to the radar's illuminationfrequency and amplifies the waveform to a level usable by transmitter340. Exciter 320 activates transmitter 340 to initiate the emission ofthe radar pulses which are fed through to radar antenna 350. Exciter 320also supplies clock signals and local oscillator signals to otherportions of the radar system. FIG. 21 illustrates a functional diagramof exciter 320 according to the present invention. Exciter 320 includesa reference oscillator 362 coupled to a calibration target generator 364through a Doppler tone generator 366, a 40 MHz clocks generator 368; andto a transmit drive generator 370. Transmit drive generator 370 iscoupled to a phase modulation section 372 and to a local oscillatorgenerator 374.

Phase modulation section 372 of exciter 320 outputs a transmitter drivesignal to transmitter 340. Exciter 320 outputs a transmitter drivesignal with a 9400 MHz center frequency within a ±80 MHz band which istransmitted to transmitter 340 at +12 dBm, nominal. Calibration targetgenerator 364 of exciter 320 outputs a 9400±80 MHz calibration signal tothe microwave integrated circuit (not shown) behind antenna 350 at -50dBm ±30 dBm, nominal. Local oscillator generator 374 outputs a signal totransmit drive generator 370 and four local oscillator outputs. Thelocal oscillator signals are output to radio frequency receiver 390 andintermediate frequency receiver 400 as shown in Table 6.

40 MHz clocks generator 368 supplies a differential output signal totiming generator 310 and to general purpose processor 380. Timinggenerator 310 transmits a first differential input signal to controlphase modulator 372 and a second differential input signal to controlpulse repetition interval phase section (not shown) of phase modulator372. A third differential input signal from timing generator 310 toexciter 320 enables a wide transmit drive gate and a fourth differentialinput signal from timing generator 310 to exciter 320 enables a narrowtransmit drive gate. Another differential input signal from timinggenerator 310 to exciter 320 enables a calibration signal switch (notshown). The frequency control word is an 8-bit parallel word fromgeneral purpose processor 380. General purpose processor 380 outputs an8-bit parallel Doppler tone control word to Doppler tone generator 366.The calibration signal attenuation word is also an 8-bit parallel wordfrom general purpose processor 380. The present invention may bepracticed using any form of logic, including, for example but notlimited to, transitor-transitor logic, CMOS logic or BiCMOS logic.According to one embodiment, the present invention utilizestransitor-transitor logic (TTL). In a preferred embodiment of thepresent invention, exciter 320 conforms to the performance parameters ofTable 7.

1.8.5 Receiver Functional Diagram

The receiver of the preferred embodiment is a multiple-conversionsuperhetrodyne receiver. Other receiver designs, known to those of skillin the art, may be used. According to one embodiment of the invention,the receiver chain includes dual channel radio frequency receiver 390and intermediate frequency receiver 400 where the signal andinterference are amplified, attenuated, and filtered at the signalfrequency. The receiver chain filters unwanted signals, especially atthe image frequency, and amplifies the signal-plus-noise to a levelwhere the noise generated in later stages does not materially contributeto the signal-to-noise ratio. The radio frequency band is centeredaround the transmit frequency, offset by the Doppler shift.

FIG. 22 is a functional diagram of radio frequency receiver 390according to one embodiment of the present invention. Radio frequencyreceiver 390 includes a power limiting section 392, a low noiseamplification section 394, an image rejection filtering section 396, anda down-conversion section 398. Power limiting section 392 may beinserted in the path from circulator/duplexer 370 to receiver 390 toprotect receiver 390 from transmitter 340.

In a preferred implementation, power limiting section 392 comprises asolid state diode limiter In a solid state diode limiter, signalsentering power limiting section 392 at a first port of an input hybridsection are divided in the input hybrid section into two equal parts,phased 90 degrees apart, and placed on two transmission lines. If thepeak-to-peak signal is less than 1.4 volt peak-to-peak, for silicondiodes, the diodes are open circuits and do not interfere with thetransmission. The signals combine at an output hybrid section. Signalcomponents greater than 1.4 volt peak-to-peak cause the diodes to shortcircuit, reflecting these components with a phase shift of 180 degrees.The reflected components are directed to a second port of the inputhybrid, which is terminated.

The output of power limiting section 392 is fed to low noiseamplification section 394, also known in the art as radio frequencyamplifier section, which amplifies the signal and interference whileintroducing minimal noise. Low noise amplification section 394 has again which effectively swamps the noise from the rest of receiver 390.The amplified signal is fed to image rejection filtering section 396which suppresses the unused or image response. Down-conversion section398 shifts the echo to an intermediate frequency by mixing incomingfrequencies with local oscillator output, LO1, frequency from exciter320 and low-pass filtering the product. Inputs to radio frequencyreceiver 390 are listed at Table 8. A preferred radio frequency receiver390 according to the invention conforms to the performance parameters ofTable 9.

1.8.6 Intermediate Frequency Receiver Functional Diagram

The majority of the signal amplification and filtering takes place inintermediate frequency receiver 400. Intermediate frequency receiver 400includes analog-to-digital (A/D) converter section 402. The inputs tointermediate frequency receiver 400 are listed in Table 10.

As mentioned above and illustrated in FIG. 23, intermediate frequencyreceiver 400 includes intermediate frequency blanking section 404coupled to a first own-conversion and gain control section 406 coupledin turn to a second down-conversion and gain control section 408 coupledto matched filter section 410 which is coupled to a coherent detector412 which is coupled to associated analog-to-digital converter section402. Down-conversion and gain control sections 406, 408 attenuate thestill strong incoming signals to prevent saturating later parts of thereceiver and shift the signal to an intermediate frequency by mixingincoming frequencies with local oscillator frequencies and low-passfiltering the product. Matched filter section 410 shapes the signals andinterference to optimize signal-to-interference ratio by admitting themaximum signal with minimum noise. The fraction of the signal powerpassed through matched filter section 410 is a function of filterbandwidth, the bandpass characteristic, and the spectrum of the receivedecho. Matched filter response is made variable if diverse waveforms, forexample, switchable pulse widths, are used. Coherent detector 412 stateswhether a target is present by comparing the signal-plus-interference toa threshold. If the signal-plus-interference, or interference alone,crosses the threshold, a detection is declared. The preferredintermediate frequency receiver 400 conforms to the performanceparameters outlined in Table 11. The preferred analog-to-digitalconverter section 402 of intermediate frequency receiver 400 conforms tothe performance parameters outlined in Table 12.

The frequency of intermediate frequency receiver 400 is selected forconvenience in building matched filter section 410 and to minimize thecontribution of the intermediate frequency stages to the overall noiselevel. The frequency of intermediate frequency receiver 400 is thedifference between signal frequency and local oscillator frequency. Thesignal is I/Q demodulated thereby translating the signal andinterference from the intermediate frequency to its information, or baseband, frequencies. I/Q demodulation recovers both the real and imaginarysignal components. The I/Q demodulated signal is a bipolar video signalwhich represents the magnitude and phase of the signal compared to thetransmit wave. Filtering is done on the composite signal in matchedfilter section 410 to optimize the signal-to-noise ratio and theresulting signal is digitized in analog-to-digital converter section402. Intermediate frequency receiver 400 outputs Sum channel digitized Iand Q signals as a 24-bit parallel word to preprocessor 420 and outputsDelta channel digitized I and Q signals as a 24-bit parallel word topreprocessor 420.

1.8.7 Timing Generator Functional Diagram

FIG. 24 illustrates the functional diagram of timing generator 310according to the present invention. Timing generator 310 includes adecoder section 312 and a signal generator 314. As noted above, timinggenerator 310 generates the frequencies and synchronization signals usedby the radar system which determines when transmitter 340 fires. Exciter320 supplies a differential 40 MHz clocks signal to decoder section 312.General purpose processor 380 outputs a 32-bit control word to decodersection 312. Timing generator 310 outputs five differential signals toexciter 320. The differential signals to exciter 320 control the phasemodulator pulse repetition interval phase, enable a wide transmit drivegate, enable a narrow transmit drive gate, and enable a calibrationsignal switch. Timing generator 310 outputs a differential signal totransmitter 340 to control the transmit pulse. Timing generator 310includes a 1 MHz or 10 MHz analog-to-digital sample clock which outputsa differential signal to intermediate frequency receiver 400. Timinggenerator 310 outputs a differential intermediate frequency blankingswitch control signal to intermediate frequency receiver 400. Timinggenerator 310 outputs a differential matched filter select switchcontrol signal to intermediate frequency receiver 400. Timing generator310 outputs a differential pulse repetition frequency (PRF) signal topreprocessor 420, to digital signal processor 420, and general purposeprocessor 380. Timing generator 310 outputs a differential dwellsynchronization signal to preprocessor 420, to digital signal processor420, and to general purpose processor 380.

1.8.8 Data Acquisition/Preprocessing Function

Signal processing in radar improves the signal-to-interference ratio andthe detection of targets in clutter, and extracts target characteristicsand behavior. Signal processing performs processes which enhance theechoes from targets and suppress interference from the receiver. Signalprocessing may include, but is not limited to, signal integration,filtering and spectrum analysis, correlation, windowing, andconvolution. Signal integration sums the composite signals within thesame range bins for several hits. Filtering and spectrum analysis is afrequency domain process in which composites of target echoes andinterference are separated into their frequency, or Doppler, components.Correlation compares the signal-plus-interference to a functionsimulating a target signal. The degree of match in the correlationprocess determines whether the composite signal-plus-interference signalcontains a target echo. Windowing may be used in the correlation processand the spectrum analysis process to reduce spectral leakage errorswhich may result when processing errors spread from one bin to otherbins. Convolution provides flexibility in some signal processes becauseconvolving in either the frequency domain or the time domain has thesame effect as multiplication in the other domain.

FIG. 25 is a functional diagram of preprocessor/digital pulse compressor420 according to the present invention. Pulse compression section 422includes a matched filter function which correlates the echo wave with adelayed copy of the transmitted signal. Prefilter section 424 generatesthe desired matched filter bandwidth. Data storage section 426 performsa data storage function which provides a place to keep digitized signalstemporarily while all the signals for a particular process are gathered.Pulse compression section 422 performs pulse compression from 1:1 to512:1 ratio. Prefilter section 424 performs digital prefiltering from1:1 to 16:1 ratio. Data storage section 426 provides a storage bufferfor the digital radar I/Q data supplied by intermediate frequencyreceiver 400.

As noted above, generally, range resolution of multiple targets requiresthat the individual targets be separated by at least the rangeequivalent of the width of the processed echo pulse. Range resolutionmay be enhanced using pulse compression, which eliminates the trade-offof wide pulses for the high energy needed for target detection againstnarrow pulses for range resolution. Preprocessor/digital pulsecompressor 420 performs digital pulse compression on the range gatesamples to provide enhanced range resolution. Pulse compression section422 includes matched filters to perform pulse compression on the rangegate samples by correlating the echo wave with a delayed copy of thetransmitted signal. According to one embodiment of the presentinvention, the autonomous landing guidance system includes a largenumber of range gates and long pulse compression codes. Performing pulsecompression in the frequency domain is more practical than in the timedomain. The discrete Fourier transform (DFT) computes the spectrum ofany function which is discrete, or sampled, in time. The discreteFourier transform changes time to frequency for sampled functions andthe inverse discrete Fourier transform changes frequency to time.Whether or not the function is periodic, the function's spectrum isdiscrete and periodic and the spectrum of a periodic time function.Mathematical analysis shows that multiplying the discrete Fouriertransform of two finite duration sequences and then inverse transformingthe product is equivalent to circularly convolving the equivalentperiodic sequences.

FIG. 26 is a functional diagram of the processing that performsfrequency domain pulse compression according to one embodiment of theinvention. Pulse compression section 422 includes code coefficientstorage section 428 coupled to zero fill function 430 and a first fastFourier transform function 432. Pulse compression section 422 alsoincludes intra-pulse motion compensation function 434 coupled to a dataformatting function 436 and a second fast Fourier transform function438. Both first and second fast Fourier transform function 432, 438 arecoupled to a multiplier function 440. The output of multiplier function440 is fed to an inverse fast Fourier transform function 442 beforebeing input to digital prefilter section 424.

According to one embodiment of the present invention, digital pulsecompression function 422 has dual channel capability, Sum and Delta,with a maximum data rate of 10 MHz. The maximum number of range samplesis 4096 per channel. Each range sample is a complex word with a 16-bitreal component and a 16-bit imaginary component. The code length isvariable from 1 to 512 complex words. When complementary code pulsecompression is used, digital pulse compression function 422 alternatesbetween two codes, code A and code B, at the pulse repetition interval,for example, code A, code B, code A, code B. Digital pulse compressor422 has the capability to store two codes at the same time whicheliminates the need to download a code each pulse repetition interval.The code coefficients are complex words, 16-bit real and 16-bitimaginary, output by general purpose processor 380 and updated at verylow rate, for example, once every few minutes, to compensate foramplitude and phase error changes as a function of temperature. Zerofilling function 430 extends the corrected code sequence to the fastFourier transform size by zero filling. The size of fast Fouriertransform function 432 is variable from 64 to 4096. Fast Fouriertransform function 432 then performs a fast Fourier transform on thepadded, or extended, code sequence. General purpose processor 380 alsoprovides the necessary information needed to perform intra-pulse phasecorrection on the radar data. After motion compensation, data formattingfunction 436 formats the radar data. The data is then zero filled andsegmented to the proper size in order to perform fast Fourier transformfunction 438. Multiplication function 440 then cell-by-cell multipliesthe code fast Fourier transform and the range sample fast Fouriertransform. An inverse fast Fourier transform of the product is thentaken to generate the pulse-compressed time sequence.

Digital prefilter 424 synthesizes matched filters for the various radarapplications. FIG. 27 is a functional diagram of digital prefilter 424according to the present invention. Digital prefilter 424 has a dualchannel capability, Sum and Delta, with a maximum data rate of 10 MHz.The maximum number range samples is 4096 per channel. Digital prefilter424 receives filter coefficients from the general purpose processor. Thefilter coefficients are floating point words down-loaded from generalpurpose processor 380. The maximum number of filter coefficients, thedown-sampling ratio, is sixteen. Digital prefilter 424 performsselectable range gate data down-sampling from 1:1 to 16:1 and stores thedata into a data buffer.

FIG. 28 is a functional diagram of power accumulation and saturationcount function 444 according to the invention. Power accumulation andsaturation count function 444 generates data which is used by theautomatic gain control algorithm to prevent excessive numbers ofsaturations and to set the received signal at an optimum level. Poweraccumulation and saturation count function 444 accumulates the totalpower from all range gates and counts the number of saturations duringeach radar dwell. The resultant saturation count data and poweraccumulation data are output to general purpose processor 380.

2.0 Optional Additional Features and Embodiments

Additional features and functions may be optionally added to or used inconjunction with the automonous landing guidance system of the presentinvention to enhance the performance or provide additional capabilitiesto the system. However, the additional features described herein neednot be limited to use with such a system and may be used in connectionwith other radar systems to enhance performance and provide additionalcapalitites.

2.1 Monopulse Beam Sharpening

As discussed in Section 1.6.3 above, monopulse beam sharpening may beused to increase the resoluting radar targets with the beam. Accordingto one embodiment of the invention, the autonomous landing guidencesystem utilizes the monopulse beam sharpening technique described below.Improved target resolution is useful not only for resolving targets inconjunction with the autonomous landing guidance system, however, but isuseful in all radar applications. Such applications may include but arenot limited to on boresight mapping, target detection andclassification, and radar real beam image sharpening. The monopulse beamsharpening function of the present invention creates a new distributedmonopulse channel, Delta₋₋ D, which improves resolution of multipletargets in the beam. According to one aspect of the invention, theinvention distinguishes closely spaced targets by time multiplexingtraditional monopulse channels with the new Delta₋₋ D channel to providemore accurate angle measurement for targets in each side of the beam.The amount of improvement is a function of the signal-to-noise ratio.

FIG. 29 illustrates the traditional Sum and Delta patterns for a typical30-inch flat plate antenna array with a 3.2 degree beam width used inweather radar. Nearly all current radar applications utilize a Sumchannel 502 and a Delta channel 504 exclusively. Sum channel (SS) 502 isobtained by transmitting and receiving through the Sum port:

    SS=SUM.sub.t *SUM.sub.r                                    Eq (27)

where:

SUM_(t) =transmission through the Sum port; and

SUM_(r) =receiving through the Sum port.

The Delta channel (SD) 504 is obtained by transmitting through the Sumport and receiving through the Delta port:

    SD=SUM.sub.t *DELTA.sub.r                                  Eq (28)

where:

SUM₁ =transmission through the Sum port; and

DELTA_(r) =receiving through the Delta port.

FIG. 29 also illustrates the Delta² or DD pattern 506, which is obtainedby transmitting and receiving through the Delta port:

    DD=DELTA.sub.t *DELTA.sub.r                                Eq (29)

where:

DELTA_(t) =transmission through the Delta port; and

DELTA_(r) =receiving through the Delta port.

In the new distributed monopulse channel according to the presentinvention, Delta₋₋ D and the Sum and Delta channel interelationships,are illustrated in FIG. 29. The new Delta₋₋ D channel 514 is obtained bymanipulation of the traditional Sum channel 510 and Delta channel 512.Delta₋₋ D 514 is the square root of the absolute value of the product ofthe Sum squared channel and the Delta squared channel:

    Delta.sub.-- D=|S.sub.t S.sub.r ×D.sub.t D.sub.r |.sup.1/2                                        Eq (30)

where:

S_(t) =transmission through the Sum port;

S_(r) =receiving through the Sum port;

D_(t) =transmission through the Delta port; and

D_(r) =receiving through the Delta port.

As illustrated in FIG. 30, Delta₋₋ D channel 514 and the absolute valueof Delta channel for a point target having a high signal-to-interferenceratio are nearly identical, except for the null depth 515. Delta₋₋ Dchannel 514 null depth is more sensitive to signal-to-interference thanthe Delta channel 512. The effect of this greater sensitivity isdiscussed later in this specification.

In practice, the Delta₋₋ D data collection is time multiplexed with thetraditional Sum and Delta channels. It is known to those of skill in theart that time multiplexing of radar channels results in only minordegradation provided the sampling rate is small, thepulse-repetition-frequency is high, such that very little decorrelationcan occur.

FIG. 31 illustrates the usefulness of the distributed monopulse channel,Delta₋₋ D, when two targets appear simultaneously in the beam. FIG. 31shows the Sum channel 516, the absolute value of the Delta channel 518,and the Delta₋₋ D channel 520 for two identical in-phase point targetsseparated by 2 degrees. Sum channel 516 and absolute value of Deltachannel 518 show typical characteristics for two point targets. Sumchannel 516 shows some broadening but no distinction between the twotargets. Absolute value of Delta channel 518 shows some broadening, anull at the mid point between the two targets, but shows no separationbetween the two targets. Rather, both Sum channel 516 and absolute valueof Delta channel 520 show patterns for two targets which are verysimilar to the patterns for a single point target, as illustrated inFIG. 31. In comparing FIG. 30 and FIG. 31, Delta₋₋ D distributedmonopulse channel pattern 520 for two point targets is visibly differentfrom Delta₋₋ D distributed monopulse pattern 514 for a single pointtarget. The present invention exploits this difference to distinguishbetween a point target and a complex distributed target.

FIG. 32 is a vector representation of the two-point target case. Asillustrated, transmitting and receiving on the Sum channel 522 resultsin a vector equal to 3.41*(T₁ +T₂); transmitting on the Sum channel andreceiving on the Delta channel 524 results in a vector equal to 1.41*(T₂-T₁); and transmitting and receiving on the Delta channel 526 results ina vector equal to 0.58*(T₁ +T₂).

In FIG. 33a-b, monopulse angle measurement techniques demonstrate theuse of the Delta₋₋ D channel information. The off-boresight angle 528 ofa target is directly proportional to the ratio: DS*!/SS* as given by Eq(13).

An off-boresight angle utilizing the Delta₋₋ D, or distributedmonopulse, channel information is computed according to: ##EQU9## where:θ=distributed off-boresight angle.

FIG. 34 illustrates that the magnitude of the two off-boresight angles,the traditional monopulse 530 and the distributed monopulse 532 areidentical for a single point target.

FIG. 35 illustrates the two off-boresight angles, the traditionalmonopulse 534 and the distributed monopulse 536, for the case of twoidentical in-phase targets that are separated by 1 degree. Thedifference between the two curves 534, 536 is directly proportional tothe separation between targets that are on the antenna right side 538and the antenna left side 540. At zero boresight, off-boresight angleutilizing traditional monopulse channel 534 is zero, while off-boresightangle utilizing the Delta₋₋ D, or distributed monopulse, channel 536shows the actual angle from the center point to each target. In theexample shown in FIG. 35 the actual angle from the center point to eachtarget is 0.5 degrees. A composite off-boresight angle α is derived fromthe two off-boresight angles according to:

    α=2×μ-θ*sign μ                     Eq (32)

where:

μ=off-boresight angle utilizing traditional monopulse channel;

θ=off-boresight angle utilizing the Delta₋₋ D channel; and

sign μ=sign of μ.

Note that the sign of the angle is transferred from μ to θ. Thiscomposite off-boresight angle is used for image sharpening.

According to the prior art, various monopulse sharpening techniques havebeen used to improve radar image quality. However, no previoustechniques have actually improved resolution capability. The monopulsebeam sharpening technique practiced according to one embodiment of thepresent invention uses monopulse angle measurements to syntheticallyenhance the image by emphasizing strong targets and highlightingtransition points. FIG. 7, discussed in detail above, illustrates themonopulse beam sharpening technique according to one embodiment of thepresent invention. The monopulse beam sharpening invention divides thescanned space into small angular bins 218, 220, 222, . . . on the orderof 0.3 degrees or less. Target intensity is then increased in theappropriate bin based on the number of monopulse angle measurementsmade. The number of times each target is detected in a particularangular bin, for example angular bin 230, is used to boost that target'sintensity level.

As noted above, FIG. 10 is an illustrative example comparing real-beamimage 232 with traditional monopulse sharpened image 234 for a singlepoint target. The sharpening capability is directly proportional to themonopulse angle measurement accuracy, which is a function ofsignal-to-interference ratio. A 15 dB signal-to-interference ratio isused in the illustrative example of FIG. 10. Simulation analysis showsthat a 15 dB signal-to-interference ratio provides approximately a 10:1sharpening improvement over real-beam for a point target.

FIG. 36 is an illustrative example comparing a real-beam image 539 witha traditional monopulse sharpened image 541 for the case of two equalsize in-phase targets spaced apart by 1 degree. As illustrated, thetraditional monopulse angle measurement is unable to separate the twotargets and may actually distort the true image. Thus, the traditionalmonopulse alone has limited capability in resolving closely spacedtargets.

FIG. 37 illustrates the effect of combining the traditional monopulseand the distributed monopulse according to the monopulse beam sharpeningmode invention 542 compared with a real-beam image 544. In theillustrative example of FIG. 37, two equal size in-phase targets spacedapart by 1 degree are clearly resolved using a 3.2 degree beam-width.Simulation analysis has shown that the resolution capacity isproportional to the signal-to-interference ratio. Asignal-to-interference ratio in the range of 10 dB to 13 dB resolves thetwo equal size in-phase targets spaced apart by 1 degree. Highersignal-to-interference allows resolution of targets spaced by less than1 degree.

FIG. 38 illustrates the effect of combining the traditional monopulseand the distributed monopulse according to the monopulse beam sharpeningmode invention for two identical targets spaced apart by 1 degree and 90degrees out-of-phase. FIG. 34 compares the image 546 which results fromcombining the traditional monopulse and the distributed monopulse with areal-beam image 548. In an illustrative example, two identical targetsspaced apart by 1 degree and 90 degrees out-of-phase are clearlyresolved. In another illustrative example (not shown), two identicaltargets spaced apart by 1 degree and being 180 degrees out-of-phase areperfectly canceled. Therefore, no sharpening improvement throughcombining the traditional monopulse and the distributed monopulse isevident.

2.2 Terrain and Obstacle Detection Warning Mode

The terrain and obstacle detection capabilities of the present inventionmay additionally be used to reduce the incidence of controlled flightinto terrain accidents and prevent other types of collisions. Thisalerting mode may be used separately or in conjunction with the othersystems such as ground proximity warning devices. Existing goundproximity warning devices either differentiate radio altitude to detectabnormal closure sites with the ground and/or use a terrain data baseand aircraft position to predict potentially hazardous situations.Unlike those ground proximity warning systems, the radar device of thepresent invention "sees" the terrain and/or obstacles actually presentahead of the aircraft. Thus, the terrain warning and alerting functionof the present invention brings additional capabilities to theprevention of CFIT accidents. For example, the present invention canwarn of potential collisions with moveable or temporary objects notlikely to be detected in prior art ground proximity devices. Theinvention provides terrain and obstacle identification and detection byproviding accurate terrain height measurements along a narrow flightcorridor and coarse terrain and obstacle measurements within a wideazimuth sector centered on the aircraft's velocity vector. The terrainand obstacle detection warning mode according to the present inventionis implemented using either a dedicated radar or an existing weatherradar. The compatibility of the terrain and obstacle detection warningmode of the present invention, with current weather radar systemsprovides the function at reduced cost and weight over implementationsusing a separate dedicated terrain radar.

The terrain and obstacle detection warning mode uses the coherentmonopulse waveform to measure top of terrain height and provide azimuthsharpened ground map images ahead of the aircraft. Frequency agility andlocal oscillator bi-phase modulation is used to improve measurementaccuracy, reduce interference from multiple time around echoes, andreduce interference from other radars.

After resolving obstacles and terrain present within the radar beam, thepresent invention further processes the radar image data to determinewhether a warning or alert of a hazardous condition should be asserted.To detect a potentially hazardous condition, the relationships of theradar detected targets to clearance planes, centered along theaircraft's velocity vector, are evaluated according to defined criteria.

The paragraphs below describe in greater detail: the construction of theclearance planes and associated alert logic; the radar beam signalprocessing for extraction of terrain data; and the hardwareimplementation of a radar according to the teachings of the presentinvention.

2.2.1 Terrain Hazard Detecting and Alerting

The terrain and obstacle detection warning mode provides early warningof obstacles in the aircraft's flight path which project above apreselected clearance plane. According to one embodiment of the presentinvention, the terrain and obstacle detection warning mode has maximumheight measurement error on the order of ±300 feet at a range of 5nautical miles; a probability of detection, or making an anglemeasurement, on the order of 99.99% and probability of false alarm,angle measurement noise, on the order of 1 in 1 million.

FIG. 39a-c illustrate antenna scan patterns 710a, 710b, the constructionof clearance plane 712, and buffer zone 714 according to the invention.Antenna scan pattern 710a is a 30 degree azimuth scan 716 along theaircraft's velocity vector 718. Antenna scan pattern 710b illustrates a30 degree elevation scan 720 scanning, for example, between +5 degreesabove horizontal 722 and -25 degrees below horizontal 724 along theaircraft's velocity vector. The combination of the azimuth and elevationscans form a three dimensional wedge of radar data.

According to the invention, a clearance plane of predefined dimensionsis virtually located within the wedge of radar data. In a preferredembodiment, clearance plane 712, also centered along the aircraft'svelocity vector 718, is on the order of 3 degrees wide and is selectablein the range 726 from about 0 feet below the aircraft's flight path toabout 1,000 feet below the aircraft's flight path and has a defaultvalue on the order of 500 feet below the aircraft's flight path.Clearance plane 712 extends to a minimum range of 10 nautical milesahead of the aircraft.

A buffer zone 714 operates within an azimuth arc on the order of 30degrees centered along aircraft velocity vector 718 and having a secondvariable clearance plane 728. Clearance plane 728 is at least 3,000 feetbelow the aircraft flight path when the aircraft is operating at analtitude above 10,000 feet and is on the order of 1,000 feet below theaircraft's flight path when the aircraft is operating at or below analtitude of 10,000 feet. These later dimensions for clearance plane 728assume the terrain and obstacle detection warning function isde-asserted at altitudes below about 1,200 feet to prevent nuisancewarnings. Buffer zone 714 extends to a minimum range of 10 nauticalmiles ahead of the aircraft.

Obstacles that are detected within the defined buffer zone areidentified and displayed on a display in a plan position indicatorformat. Displayed obstacles may be merged on the display with weatherdata using different colors as is well known to those of skill in theart, for example, see U.S. Pat. No. 3,369,231.

When an obstacle is detected above the clearance plane, the inventionissues a warning which may be an aural and/or visual alert. The rangeand height of the highest obstacle above the clearance plane within aradar range segment on the order of 1 mile is displayed in alphanumericformat. The invention may limit the maximum number of displayed targetsabove the clearance plane to prevent over crowding of the displayeddata. For example, the invention may limit the maximum number ofdisplayed targets above the clearance plane to 10.

FIG. 40a-c illustrate the terrain and obstacle detection warning modedisplay format according to one embodiment of the present invention ingreater detail. Those of skill in the art will recognize that otherdisplay formats may be used to display the terrain and obstacledetection warning mode information. When operating in a weather displaymode, weather radar data is displayed in a standard weather displayformat 730a. When operating in a first weather and terrain display mode,display 730b displays horizontal or azimuth scan data and an aircraftvelocity vector indicator 732. The azimuth scan data contains limitedamounts of elevation data in wide view. Also displayed are indicationsof the terrain data relative to the buffer zone. As shown in FIG. 40,the display includes and an indication that detected terrain lies abovea selected buffer zone using an "above buffer zone" indicator 734; anindication that detected terrain lies below a buffer zone using a "belowbuffer zone" indicator 736; and an indication that the terrain positionrelative to the buffer zone is unknown using an "unknown position"indicator 738.

When operating in a second weather and terrain display mode, a secondweather and terrain display 730c includes vertical, or elevation, scandata in addition to the azimuth scan data. Display 730c provides anaircraft velocity vector indicator 732 and an indication that detectedterrain lies above a selected buffer zone using an "above buffer zone"indicator 734; an indication that detected terrain lies below a bufferzone using a "below buffer zone" indicator 736; and an indication thatthe terrain position relative to the buffer zone is unknown using an"unknown position" indicator 738. Additionally, second weather andterrain display 730c provides an indication of the location of thedetected terrain relative to the selected clearance plane. An "aboveclearance plane" indicator 740 indicates a potential hazard. In oneembodiment of the invention, range from the aircraft 742 and distancebelow the aircraft 744 are displayed for terrain lying above clearanceplane 728.

2.2.2 Interleaving of Weather and Terrain Detection Modes

The mode of operation for the radar of the present invention may beselected in various ways. For example, the pilot may select that theradar operate purely in the mode of collecting terrain data, or purelyin the mode of collecting weather data, or an autonomous landingguidance mode. The mode may also be automatically controlled based onaircraft altitude, detection of a weather or terrain threat, or aircraftconfiguration. Preferably, however, the radar operates to interleaveboth weather and terrain data. This interleaved mode of operationpermits the pilot to select a single mode for display if desired, yetretains the simultaneous weather avoidance and terrain hazard detectioncapabilities of the present invention.

FIG. 41 illustrates one method of interleaving weather and terrain datacollection according to the present invention. In the embodiment of FIG.41, a first radar scan 696 sweeps through an azimuth angle of -90 to +90degrees and collects only weather data. During a second return scan 698,weather data collection 698a is interrupted at a point 15 degrees inadvance of the aircraft's velocity vector. At this point, the radarbegins collecting terrain monopulse elevation data in wide view andazimuth data for the next 30 degrees of sweep 698b at the rate ofapproximately 30 degrees per second. During the azimuth scan, theantenna elevation angle is computed based on aircraft attitude. Notethat terrain detection sweep 698b is not centered about an azimuth angleof zero degrees, but is offset to account for the aircraft crab angle.During a third scan 706, weather data collection 706a is interrupted ata point coincident with the direction of the aircraft velocity vectorand the radar transitions to a vertical, or narrow view scan 706b. Scan706b collects a elevation data through an arc of approximately 30degrees, from +5 degrees to -25 degrees, at the rate of 30 degrees persecond. If one or more targets are detected above +5 degrees, thepositive elevation of the scan may be increased above +5 degrees untilthere is no further target detection. After vertical scan 706b, theinvention repositions the antenna at the pre-interruption position andreasserts a weather data collection mode 706c. The entire terrain datacollection and processing time is on the order of 1 second per frame.

When the invention is implemented using weather radar, compliance mustalso be maintained with current Federal Aviation Administrationregulations requiring a windshear detection capability. The windsheardetection function is typically enabled below 1,200 feet altitude, inthe vicinity of an airport during take off and landing. The interleavingof windshear and terrain data has been previously described inconnection with FIGS. 17a-b.

2.3 Radar Characteristics, Waveforms and Signal Processing

Radar requirements and characteristics for the ground proximity warningfunction are prescribed by a combination of the desired warning times,the capabilities and requirements of the weather radar function (whendual purpose use is intended), and the desired accuracy of terrainresolution. For example, according to a preferred embodiment of theinvention, the terrain look ahead ability is designed to providesufficient warning for an aircraft to clear a 10,000 foot obstacle witha maximum aircraft acceleration of 0.25 g.

FIG. 42 contains a graph of maximum vertical climb vs. warning distanceof the type useful for deriving the radar performance specificationsaccording to the present invention. As shown by curve 602, for altitudesabove 10,000 feet and a speed of 600 knots, the radar must have a 8.5nautical mile look-ahead capability to clear the 10,000 foot obstacle.As shown by curve 604, for altitudes below 10,000 feet at a speed of 250knots, the radar must have a 3.5 nautical mile look-ahead capability.

Table 13 compares the preferred radar parameters for a radar optimizedfor terrain detection and a dual purpose weather and terrain radar. Fora radar optimized for the primary purpose of terrain detection, thepreferred range bin size is 50 meters. However, larger range bin sizesare consistent with the capability of existing weather radars. Althoughadequate for terrain angle measurement, the larger bin size is toocoarse for height measurement of narrowly extended targets. As describedelsewhere in this document, measurement of narrow vertically extendedtargets, for example, radio towers, can be accomplished according tomonopulse beam sharpening techniques generally known to those of skillin the art or as described in this document.

The optimum number of range bins for terrain detection is 384. Thenumber of range bins for the radar of the present invention is selectedto cover a minimum of 10 nautical miles as determined from therequirements of FIG. 42.

Minimum clutter backscatter coefficient and clutter to noise ratio arealso significant in determining the performance and suitability of aradar to the terrain detection function. The terrain and obstacledetection warning mode, according to the present invention, operateswhen the grazing angle is very shallow, on the order of less than 3degrees. Thus, the minimum clutter backscatter coefficient is on theorder of -45 dB square meter per square meter for snow covered terrain.Weather radars also provide sufficient clutter-to-noise ratio to fullyimplement the terrain and obstacle detection warning mode of theinvention; for example, resolving terrain in 12 millimeter per hourintervening rain.

The radar waveform of FIG. 4 described previously is also suitable forpracticing the present invention. Other waveforms may be used. Signalprocessing of the radar signals to detect terrain in the groundproximity warning application occurs in the same manner as described forthe detection of terrain and obstacles when in the autonomous landingguidance mode.

Preferred embodiments of the invention have been described. Variationsand modifications will be obvious to those of skill in the art. Headingsused herein are for the convenience of the reader and do not have legalmeaning. For example, the present invention may also be used to provideobstruction and obstacle clearance, as well as intrustion detectionduring taxi and takeoff in addition to landing. The invention also hasapplications to marine navigation and uses for marine weather radar.Furthermore, terrain alterting algorithms as disclosed in U.S. Pat. No.4,646,244 and copending U.S. application Ser. No. 08/509,642 may be usedto generate warnings of potential flight into the terrain detected bythe present invention. For at least these reasons, the invention is tobe interpreted in light of the claims and is not limited to theparticular embodiments described herein.

                  TABLE 1                                                         ______________________________________                                        Radar Parameter    Value                                                      ______________________________________                                        Pulse repetition frequency                                                                       6 kHz                                                      Number of range bins                                                                             512                                                        Range bin size     15 meters                                                  Transmit pulse width                                                                             1.6 microseconds                                           Digital pulse compression                                                                        16:1 complementary codes                                   Radio frequency bandwidth                                                                        160 MHz                                                    Number of frequencies                                                                            32                                                         Number of pulse repetition                                                                       128                                                        intervals                                                                     ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Radar Parameter      Value                                                    ______________________________________                                        Peak power           160 watts                                                Antenna gain (one way)                                                                             35 dB                                                    Beamwidth            3.4 degrees                                              Radio frequency bandwidth                                                                          160 MHz, minimum                                         Range bin size       15 meter                                                 Pulse compression ratio                                                                            1:1, 16:1                                                Coherent integration gain                                                                          2:1                                                      Frequency step size  5 MHz                                                    Frequency sub-bands  4                                                        Number of frequencies per                                                                          8                                                        sub-band                                                                      Transmit pulse width 10 nanoseconds, 1.6                                                           microseconds                                             Matched filter bandwidth                                                                           10 MHz                                                   Noise figure         5 dB, maximum                                            Radio frequency losses                                                                             4 dB                                                     Rain attenuation for 12                                                                            0.285 dB/kilometer                                       mm/hour (one way)                                                             Number of range bins 512                                                      ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Antenna Positioner Parameter                                                                     Performance                                                ______________________________________                                        Position accuracy  0.1 degree                                                 Pointing stability 0.1 degree                                                 Antenna phase center                                                                             0.15 millimeter, maximum                                   displacement relative to pedestal                                             mounting surface                                                              Azimuth to elevation coupling                                                                    0.3%, maximum                                              Weight             35 pounds, maximum                                         ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        Antenna Array Parameter                                                                           Performance                                               ______________________________________                                        Center frequency    9400 MHz                                                  Radio frequency bandwidth                                                                         160 MHz                                                   Beam shapes         Selectable between                                                            pencil and fan                                            Radio frequency ports                                                                             Sum, Delta elevation, and                                                     Delta azimuth                                             Gain over radio frequency                                                                         35 dB, nominal                                            bandwidth                                                                     Gain variation over radio                                                                         0.5 dB, maximum                                           frequency bandwidth                                                           Beamwidth characteristics:                                                    Azimuth             3.4 degrees, nominal; 3.6                                 Elevation (pencil)  degrees, max                                              Elevation (fan)     3.2 degrees, nominal; 3.4                                                     degrees, max                                                                  Cosecant squared cosine                                   Sidelobe characteristics:                                                     Sum channel sidelobes:                                                        Peak                -30 dB                                                    RMS                 -38 dB                                                    Delta channel sidelobes:                                                      Peak                -28 dB                                                    Voltage standing wave ratio                                                                       1.5:1                                                     (VSWR)                                                                        Polarization        Horizontal                                                Delta channel null depth                                                                          -25 dB, minimum                                           Delta lobe imbalance                                                                              0.5 dB, maximum                                           Port-to-port isolation                                                                            25 dB, minimum                                            Mechanical-to-electrical boresight                                                                0.1 degree, maximum                                       alignment error                                                               Sum-to-Delta imbalance (over                                                  radio frequency bandwidth and                                                 temperature range):                                                           phase imbalance (0 to 180                                                                         +/-10 degrees, maximum                                    degrees)                                                                      gain imbalance      0.5 dB, peak-to-peak                                      ______________________________________                                    

                  TABLE 5                                                         ______________________________________                                        Transmitter Parameter                                                                             Performance                                               ______________________________________                                        Peak power          160 watts                                                 Duty cycle          15%, minimum                                              Radio frequency bandwidth                                                                         160 MHz                                                   Radio frequency center frequency                                                                  9400 MHz                                                  Intra-pulse and phase stability:                                              linear              0.5 dB, 10 degrees peak-                                  sinusoidal          to-peak                                                                       0.3 dB, 3.6 degrees peak-                                                     to-peak                                                   random              0.58 dB, 4.0 degrees                                                          RMS                                                       single frequency    0.02 dB, 0.23 degrees                                                         peak-to-peak                                              over the radio frequency                                                                          0.3 dB, 3.6 degrees peak-                                 bandwidth           to-peak                                                   Pulse characteristics:                                                        width               100 nanoseconds to 512                                                        microseconds                                              rise and fall time  10 nanoseconds                                            Voltage standing wave ratio                                                                       1.6:1                                                     (VSWR)                                                                        ______________________________________                                    

                  TABLE 6                                                         ______________________________________                                        Local                                                                         Oscillator                                                                              Output signal  Destination                                          ______________________________________                                        LO1       8720 MHz +/- 80 MHz                                                                          Radio frequency receiver                                                      290                                                  LO2       520 MHz +/- 80 MHz                                                                           Intermediate frequency                                                        receiver 300                                         LO3       180 MHz +/- 80 MHz                                                                           Intermediate frequency                                                        receiver 300                                         LO4       dual phase,    Intermediate frequency                                         reference/-90  receiver 300                                                   degrees, 20 MHz at                                                            +10 dBm                                                             ______________________________________                                    

                  TABLE 7                                                         ______________________________________                                        Exciter Parameter  Performance                                                ______________________________________                                        Transmit drive:                                                               Frequency          9400 MHz +/- 80 MHz                                        Accuracy           0.05%                                                      Power out          +12 dB, nominal                                            Number of frequencies                                                                            32 (5 MHz) discrete                                                           output frequencies,                                                           digitally programmable                                     Step size accuracy 750 Hz periodic error;                                                        500 Hz RMS random                                                             error                                                      Setting time:                                                                 single step jump   30 microseconds,                                                              maximum                                                    full band jump     60 microseconds,                                                              maximum                                                    Rise/fall times for 10% to 90%                                                detected radio frequency:                                                     at narrow gate     10 nanoseconds                                             output                                                                        at wide gate output                                                                              300 nanoseconds                                            Switch ON/OFF ratio:                                                          narrow gate        50 dB, minimum                                             wide gate          50 dB, minimum                                             combination        113 dB, minimum                                            Bi-phase modulation                                                                              Reference phase and                                                           180 degrees with +/-2.5                                                       degrees phase accuracy.                                                       Switching speed: 10                                                           nanoseconds, maximum                                       Spurious:                                                                     radio frequency +/- n (20                                                                        -60 dBc, maximum, for n =                                  MHz)               1, 2, . . . 34                                             all others         -50 dBc                                                    Phase noise:                                                                  100 Hz             -107.5 dBc/Hz                                              3 kHz              -122 dBc/Hz                                                100 KHz            -140 dBc/Hz                                                Intra-pulse phase stability:                                                  short pulse (≦1 microsecond)                                                              0.115 degree, peak-to-                                                        peak                                                       long pulse (>1 microsecond)                                                                      3.6 degrees, peak-to-                                                         peak                                                       Inter-pulse short term phase                                                  stability:                                                                    single frequency   0.115 degree, peak-to-                                                        peak                                                       over radio frequency                                                                             3.6 degrees, peak-to-                                      bandwidth          peak                                                       Short term frequency stability                                                                   +/-2.6 Hz                                                  First local oscillator:                                                       frequency          9400 MHz +/- 80 MHz                                        accuracy           0.05%                                                      power out          +10 dBm +/- 2 dB                                           spurious:                                                                     LO1 +/- 680 Mhz    -113 dBc                                                   all others         -60 dBc                                                    Second local oscillator:                                                      frequency          520 MHz                                                    accuracy           0.05%                                                      power out          +10 dBm +/- 2 dB                                           spurious:                                                                     LO2 +/- 200 MHz    -83 dBc, maximum                                           all others         -60 dBc, maximum                                           short term frequency                                                                             +/-2.6 Hz                                                  stability                                                                     Third local oscillator:                                                       frequency          180 MHz                                                    accuracy           0.05%                                                      power out          +10 dBm +/- 2 dB                                           spurious:                                                                     LO3 +/- 20 MHz     -51 dBc, maximum                                           all others         -50 dBc, maximum                                           short term frequency                                                                             +/-2.6 Hz                                                  stability                                                                     Fourth local oscillator                                                       frequency          20 MHz                                                     accuracy           0.05%                                                      power out          +10 dBm +/- 2 dB                                           spurious:          -60 dBc, maximum                                           short term frequency                                                                             +/-2.6 Hz                                                  stability                                                                     40 MHz clock:                                                                 accuracy           0.05%                                                      duty cycle         50%, nominal, TTL                                          frequency stability                                                                              +/-2.6 Hz                                                  Phase lock indicator                                                                             ON/OFF                                                     Voltage standing wave ratio                                                                      1.5:1                                                      (VSWR)                                                                        ______________________________________                                    

                  TABLE 8                                                         ______________________________________                                        Radio                                                                         Frequency                                                                     Receiver Input                                                                Signal Source                                                                             Input Signal Signal Parameters                                    ______________________________________                                        Antenna 250 Radio frequency                                                               signal:                                                                       center frequency                                                                           9400 MHz                                                         tolerance band                                                                             +/-80 MHz                                                        range        -109 dBm to 10 dBm                                   Exciter 220 Local oscillator,                                                                          8720 MHz                                                         LO1, signal:                                                                  center frequency                                                                           +/-80 MHz                                                        tolerance band                                                                             -10 dBm, average                                                 range                                                             ______________________________________                                    

                  TABLE 9                                                         ______________________________________                                        Radio Frequency Receiver                                                      Parameter          Performance                                                ______________________________________                                        Noise figure at operational                                                                      4.5 dB, maximum                                            gain                                                                          Radio frequency limiting                                                                         50 dB, minimum                                             Radio frequency switching                                                                        500 nanoseconds, maximum                                   speed                                                                         Maximum input power without                                                                      +30 dBm, maximum                                           damage                                                                        Image rejection filter                                                                           500 MHz, 6-pole, LC                                                           realization,                                                                  Gaussian to 6 dB                                           Voltage standing wave ratio                                                                      1.5:1                                                      (VSWR)                                                                        ______________________________________                                    

                  TABLE 10                                                        ______________________________________                                        Intermediate                                                                  Receiver Input              Input signal                                      Signal        Signal parameters                                                                           source                                            ______________________________________                                        Intermediate  680 MHz signal at -                                                                         Radio frequency                                   frequency input                                                                             89 dBm to +30 receiver 290                                                    dBm, average                                                    Local oscillator                                                                            520 MHz signal at                                                                           Exciter 220                                       signal, LO2   +10 dBm, nominal                                                Local oscillator                                                                            180 MHz signal at                                                                           Exciter 220                                       signal, LO3   +10 dBm, nominal                                                Local oscillator                                                                            20 MHz signal at                                                                            Exciter 220                                       signal, LO4   +10 dBm, nominal                                                Blanking switch                                                                             differential logic                                                                          Timing generator                                  control       signal        210                                               Matched filter select                                                                       differential logic                                                                          Timing generator                                  switch control                                                                              signal        210                                               Attenuation control                                                                         8-bit parallel                                                                              General purpose                                                 differential control                                                                        processor 280                                                   word                                                            I/Q phase correction                                                                        16-bit parallel                                                                             General purpose                                                 differential I/Q                                                                            processor 280                                                   phase correction                                                              word                                                            I/Q amplitude 16-bit parallel                                                                             General purpose                                   correction    differential I/Q                                                                            processor 280                                                   amplitude                                                                     correction word                                                 ______________________________________                                    

                  TABLE 11                                                        ______________________________________                                        Intermediate Receiver                                                         Parameter          Performance                                                ______________________________________                                        Noise figure       12 dB, maximum, at                                                            operation gain,                                                               where: operational gain is                                                    that gain setting which                                                       produces 2 q noise at the                                                     analog-to-digital converter                                                   with radio frequency receiver                                                 noise as a source.                                         Intermediate frequency blank:                                                 On/Off ratio       40 dB, minimum;                                            switching speed    500 nanoseconds, maximum                                   Maximum input power without                                                                      +50 dB                                                     damage                                                                        Transmit leakage recovery                                                     time:                                                                         recovery time to 1 dB pre-                                                                       200 nanoseconds,                                           saturation level   maximum,                                                   recovery time to full pre-                                                                       500 nanoseconds,                                           saturation Level   maximum,                                                   for all pulse widths                                                          Intermediate frequency                                                        receiver gain control:                                                        minimum gain control                                                                             95 dB, minimum                                             gain control step  1 dB                                                       resolution                                                                    gain control accuracy                                                                            +/-1 dB                                                    switching speed    10 nanoseconds, maximum                                    attenuation        monotomic                                                  Intermediate frequency                                                        receiver matched filters:                                                     quantity           2 selectable matched filters,                                                 minimum                                                    type               6-pole LC realization;                                                        Gaussian to 6 dB                                           first bandwidth    10 MHz                                                     second             1 MHz                                                      bandwidth                                                                     Operating temperature                                                                            -40 to +85 degrees                                                            Centigrade                                                 Voltage standing wave ratio                                                                      1.5:1                                                      (VSWR)                                                                        ______________________________________                                    

                  TABLE 12                                                        ______________________________________                                        Analog to Digital Converter                                                   Parameter          Performance                                                ______________________________________                                        Analog-to-digital dynamic rate                                                                   12-bit offset binary                                       Number of channels 4 (SUMI, SUMQ, DELTAI,                                                        DELTAQ                                                     Analog-to-digital sampling rate:                                              minimum sampling rate                                                                            1 MHz                                                      maximum sampling rate                                                                            10 MHz                                                     Linearity          +/-0.5 q                                                   Full scale Error   +/-1 q                                                     Hysteresis         +/-0.25 q                                                  Analog-to-digital aperture jitter                                                                60 picosecond, maximum                                     I/Q images         -60 dB                                                     DC bias (after digital                                                                           ≦1/16 q                                             correlation)                                                                  Analog-to-digital bias drift                                                                     0.375 q per minute                                         ______________________________________                                    

                  TABLE 13                                                        ______________________________________                                                       Terrain Dedicated                                              Parameter      Radar        Weather radar                                     ______________________________________                                        Pulse repetition                                                                             7 kHz        7 kHz                                             frequency                                                                     Number of range                                                                              384          128                                               bins                                                                          Range bin size 50 meters    150 meters                                        Transmit pulse width                                                                         330 nanoseconds                                                                            1 microsecond                                     Radio frequency                                                                              200 MHz      24 MHz                                            bandwidth                                                                     Number of      4 × 8-frequency                                                                      8                                                 frequencies    sub-bands                                                      Number of pulse                                                                              256          256                                               repetition intervals                                                          per dwell                                                                     ______________________________________                                    

What is claimed is:
 1. A radar system, comprising:a radar antenna forradiating and receiving radar signals through a sum port and a deltaport; a radar transmitter-receiver for generating and detecting saidradar signals, said transmitter-receiver coupled to said antenna; asignal processor coupled to said antenna, said signal processorincluding:means for generating a sum squared channel, means forgenerating a delta squared channel, and means for generating a delta₋₋ dchannel.
 2. The radar system of claim 1 further comprising a displaycoupled to said signal processor, said display receiving and displayingat least one of said sum squared channel, said delta squared channel andsaid delta₋₋ d channel.
 3. The radar system as recited in claim 1,wherein the radar system is a monopulse radar.
 4. The radar system asrecited in claim 3, wherein said monopulse radar is a weather radar. 5.The radar system as recited in claim 1, wherein said monopulse radar isan X band radar.
 6. The radar system as recited in claim 5, wherein datacollection on said delta₋₋ d channel is time multiplexed with datacollection on at least one of said sum squared channel, said deltasquared channel and said delta channel.
 7. The radar system as recitedin claim 5, wherein said data collection occurs over a small definitesampling interval.
 8. The radar system as recited in claim 1, whereinsaid radar antenna scans a selected sector and said signal processorincludes intensity increasing means for increasing the intensity oftargets and for increasing the intensity of transition points betweensaid targets.
 9. The radar system as recited in claim 8, wherein saidintensity increasing means includes:means for dividing said scannedsector into a selected number of bins means for detecting each of saidtargets in each of said bins, and means for counting the number of timeseach of said targets is detected in each of said bins.
 10. The radarsystem as recited in claim 1, wherein the sensitivity of said delta₋₋ dchannel is a function of the radar signal-to-interference ratio.
 11. Amonopulse radar system, comprising:a radar antenna having a capacity forradiating and receiving radar signals through a sum port and a deltaport; a radar transmitter for generating said radar signals, saidtransmitter coupled to said antenna; a radar receiver for amplifying andfiltering said received radar signals, said receiver coupled to saidantenna; and a signal processor coupled to said antenna, said signalprocessor including:means for generating a sum squared channel, saidmeans including processing a signal transmitted and received throughsaid sum port, means for generating a delta squared channel, said meansincluding processing a signal transmitted and received through saiddelta port, and means for generating a delta₋₋ d channel, said meansincluding calculating the square root of the absolute value of theproduct of said sum squared channel and said delta squared channel. 12.The system of claim 11 further comprising a display operatively coupledto said signal processor and receiving at least one of said sum squaredchannel, said delta squared channel and said delta₋₋ d channel fordisplay.
 13. The radar system as recited in claim 11, wherein saidsignal processor further includes means for generating a delta channel,said means including processing a signal transmitted through said sumport and received through said delta port.
 14. The radar system asrecited in claim 13, wherein data collection on said delta₋₋ d channelis time multiplexed with data collection on each of said sum squaredchannel, said delta squared channel and said delta channel.
 15. Theradar system as recited in claim 11, wherein said radar antenna scans aselected sector and said signal processor includes intensity increasingmeans for increasing the intensity of targets and for increasing theintensity of transition points between said targets.
 16. The radarsystem as recited in claim 15, wherein said intensity increasing meansincludes:means for dividing said scanned sector into a selected numberof bins means for detecting each of said targets in each of said bins,and means for counting the number of times each of said targets isdetected in each of said bins.
 17. The radar system as recited in claim11, wherein the sensitivity of said delta₋₋ d channel is a function ofthe radar signal-to-interference ratio.
 18. A monopulse radar system,comprising:a radar antenna for radiating and receiving radar signalsthrough a sum port and a delta port; a radar transmitter for generatingsaid radar signals, said transmitter means operatively coupled to saidantenna means; a radar receiver for receiving a return echo from saidradiated radar signals, said receiver means operatively coupled to saidantenna means; a signal processor coupled to said antenna means, saidsignal processor means including:means for generating a sum squaredchannel, said means including processing a signal transmitted andreceived on said sum port, means for generating a delta channel, saidmeans including processing a signal transmitted on said sum port andreceived on said delta port, means for generating a delta squaredchannel, said means including processing a signal transmitted andreceived on said delta port, means for generating a delta₋₋ d channel,said delta₋₋ d generating means including means for calculating thesquare root of the absolute value of the product of said sum squaredchannel and said delta squared channel, and each of said generatingmeans including intensity increasing means for increasing the intensityof targets and for increasing the intensity of transition points betweensaid targets; and a display for displaying radar images, said displaymeans operatively coupled to said signal processor means.
 19. The radarsystem as recited in claim 18, wherein said intensity increasing meansincludes:means for dividing said scanned sector into a selected numberof bins; means for detecting each of said targets in each of said bins;means for counting the number of times each of said targets is detectedin each of said bins; and said display includes means for varying theintensity of a displayed image as a function of the number of times saidtarget is detected in said bin.
 20. A method for target resolution in amonopulse radar system, said method comprising the steps of:transmittingand receiving a radar signal through a sum port of a radar antenna;transmitting and receiving a radar signal through a delta port of aradar antenna; processing said signal transmitted and received throughsaid sum port to generate a sum squared channel; processing said signaltransmitted and received through said delta port to generate a deltasquared channel; and processing said sum squared channel and said deltasquared channel to generate a delta₋₋ d channel.
 21. The method recitedin claim 20, wherein said processing of said sum squared channel andsaid delta squared channel to generate a delta₋₋ d channel includescalculating a square root of an absolute value of a product of said sumsquared channel and said delta squared channel.
 22. The method recitedin claim 21, wherein the target resolution improvement is a function ofsignal-to-interference ratio.
 23. A method for target resolution in amonopulse radar comprising the steps of:processing a radar sum channelsignal to obtain a sum squared channel of target information; processinga radar delta channel signal to obtain a delta squared channel of targetinformation; and creating a delta₋₋ d channel of target information bycalculating a square root of an absolute value of a product of said sumsquared channel and said delta squared channel.
 24. A monopulse radarcomprising:a first input for receiving a sum channel information; asecond input for receiving a delta channel information; and a signalprocessor coupled to said first input and to said second input for:(a)creating a delta₋₋ d channel by calculating a square root of an absolutevalue of a product of a sum squared channel and a delta squared channel;and (b) resolving a target from at least one of said sum information,said delta information and said delta₋₋ d channel.